Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"There can only be two types of motion: constant and changing. There also only two types of net forces: zero and non-zero."

"Newton's third law of motion isn't necessarily about motion or net force, but the properties of force itself."

"We can use the mnemonic device 'POF-OST-ITO' in order to test if Newton's third law applies or not. POF stands for 'pair of opposite forces,' OST is 'of same type,' ITO 'involving two objects.'"

"Newton's third law relates only two forces of the same type acting on two different objects. However Newton's first law deals with two forces acting upon one object. Additionally, if the POF-OST-ITO test fails, then it is not a N3 scenario, but it is not yet safe to assume it is a N1 scenario either, until you do further investigation."

"Newton's second law shows how the acceleration depends on both the net force and the mass. The magnitude of acceleration is proportional to the net force acting on the object, and inversely proportional to the mass. The net force includes only the forces that the environment exerts on the object of interest. The kinetic friction force opposes the relative sliding motion."

"Everything is effected on by gravity. If something is touching a surface, it will have a normal force acting on it."

"The normal force can only exist when two surfaces are making contact with one another. If two objects are not making contact, then there is no normal force."

"Friction is another component when dealing with the motions of an object. It is a force that is parallel to the surface an object is moving on. Static friction is what makes an object remain stationary even when a force is applied, and only moves when the applied force is slightly greater than the maximum force of static friction."

"Static friction force comes into play when pulling on an object at rest, and if the force applied is small enough static friction will cancel out the applied force resulting in no movement."

"That an object initially at rest interacts with a surface through static friction. As the object moves, after overcoming the maximum static friction, the resistance turns into kinetic friction. Friction is a parallel and normal force is perpendicular to the surfaces in contact."

"Static friction is the friction that a surface has on a object that is resting on the surface and is directly proportional to the normal force on said object. Next, there is kinetic friction which is friction that occurs between two sliding surfaces. Also kinetic friction is typically less than the static friction."

"Static and kinetic frictional forces seem pretty straightforward. Static refers to when there is friction acting on an object but the object is not moving. Kinetic is when there is friction acting on an object but the object is moving."

"Was a little lost, so there isn't much I understand."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I feel like I understand Newton's laws from the textbook. However, when it comes to applying the concepts to a free body diagram, I start getting confused with the direction of the forces and how they cancel out especially when things are stacked on top of each other."

"I still have trouble distinguishing between Newton's first law and second law."

"I was slightly confused as to why the pulleys and ropes would have equal tensions for the different worksheet examples in class. But after further research I understood that neither the mass nor acceleration of gravity changed, therefore the tension forces stayed the same."

"I get confused on how to determine which law is being used between Newton's first law or third law. I also don't understand the difference between static and kinetic friction forces and the magnitudes associated with them."

"The difference between static friction and kinetic friction aren't clear to me."

"The coefficient of friction and what it stands for might need some clarification for me."

"I would like some more work in class with problems on kinetic friction and static friction. I understand that static friction is friction for stationary objects, like a box on the ground, and that kinetic friction is friction for moving objects, like a box sliding across the ground. However, I'm still unsure of how this would work in real-world situations."

"I don't know how to put friction in an equation or how to use it to solve for something."

"Why are there no SI units for static/kinetic friction coefficient?"

"For the most part it is pretty easy to visualize friction, since we have all experienced it before."

"Didn't really find anything confusing."

"I understood most of it conceptually, I think it could get more confusing when these concepts are applied in a problem."

"Most of it."

What is the meaning of the "normal" in the "normal force?"

"The perpendicular force applied between two objects contacting each other."

"It just means the force is perpendicular to the surface."

"The 'normal' force refers to the perpendicular direction with respect to the surface."

"Normal means 'perpendicular.'"

"There will almost always be normal force on any given objects in contact, regardless of its state or location? That's what makes it 'normal,' right?

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

" Unitless."

The coefficients because the units cancel."

"I do not believe these coefficients have units."

"Newtons?"

"kg·m²·s²?"

"I'm not entirely sure."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [77%]

An external horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [83%]

An external horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [72%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied on it, so it slows down:
f_k = µ_k·N. [21%]

An external horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [23%]

An external horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [43%]

An external horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [58%]

An external horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [32%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Can we go over the friction coefficients? Please explain static friction force and kinetic friction force in detail in class! I could use some review on the equations for the magnitudes of static and kinetic frictional force. Is the friction coefficient different between objects that do not appear smooth. For example, is the constant for ice on ice different than say, bumpy ice on ice?" (Yes, the coefficient will be larger for rougher surfaces than for smoother sliding surfaces.)

"Are there ways to increase of decrease friction?" (Change the smoothness/roughness of the surfaces, or add a lubricant (which is "smooth" on a molecular level; long hydrocarbon chains can align with each other to roll like logs) between the two surfaces.)

"What is it that causes friction? Is it simply objects hooking onto each other at microscopic levels?"

"Does static friction force become kinetic friction force once the object starts moving?" (Yes, once the object is already unstuck.)

"What I found confusing was that static friction force and the applied force on a object are directly proportional, except for the fact that static friction has a maximum amount, as opposed to applied force which can keep increasing indefinitely. If the two are directly proportional, shouldn't the static friction force increase as long as the applied force increases, or shouldn't the applied force also have a maximum if the static friction has one as well?" (You can arbitrarily exert any amount of applied force on an object, but the static friction force will have a maximum amount because at some point your applied force will "unstick" the object so it will then begin to move. I suppose if you stuck together the two surfaces magic superglue, then you could exert an infinite amount of applied force, and the static friction force would then also be infinite, provided the magic superglue still holds the object stationary.)

"I hope I'm understanding this correctly! It seems to make sense so that's either a really good sign or a really bad sign."

"Definitely need some review, this weekend fried my brain."

"I don't like this :("

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2018
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"If you have a constant speed and direction then Newton's first law applies. If you have a change in speed and/or direction then Newton's second law applies."

"Static friction goes against the impending relative motion between two objects and that kinetic friction goes against the relative sliding motion between two surfaces. Newton's second law can be used to find the acceleration of an object."

"I understand the difference between both static friction and kinetic friction. Static friction is the force needed to keep a stationary object at rest, for example when you go rock climbing. The static frictional forces help support someone's weight as they press against the walls of the rock and create large amounts of normal forces. Now once two surfaces begin sliding over one another then kinetic friction is produced. And kinetic friction is what slows down a moving object. For example, when you push a chair across the floor, the initial force to actually move the object requires greater force, but it takes less force to keep the object sliding."

"Static friction depends on the amount of force that is applied to stationary object. Kinetic friction is applied when an object is sliding across the floor."

"The difference between static and kinetic friction. Static friction is when an object is at rest, and once it starts to move it has kinetic friction acting on it."

"How to distinguish between Newton's first law and third law. Newton's first law relates two or more forces acting on the same object (if motion is constant), while Newton's third law relates the same force acting on two different objects."

"The coefficient of static friction is equal to or higher than the coefficient of kinetic friction."

"For the most part I seemed to understand what I read but I had to read it twice."

"I kind of understand the whole concept of friction forces, but it is still very confusing."

"Static friction has a maximum value and once that value is surpassed, then I think of it like the transfer of friction from static to kinetic. It's interesting that the maximum static friction value is independent of surface area, which I did not expect. However, the maximum value of static friction does depend on the type of surface materials."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I need some more practice on distinguishing between newtons first and third laws in different situations. I also need to memorize what 'POF-OST-ITO' stands for."

"Static friction force--it does not make sense to me that there can be a frictional force when the object is stationary and why tugging on the object doesn't cause it to move, but I think I understand now."

"Knowing what equations to use to solve for a static/kinetic friction problem."

"I didn't understand why the magnitude of the kinetic frictional force is proportional to the magnitude of the normal force."

"The relationship between friction and normal force."

"Difference between static and kinetic friction."

"What I found to be the most confusing from the assigned reading was the difference between static and kinetic friction. I think that static friction is applied force to keep an object stationary and then kinetic friction is when two things slide over each other, but I am not sure."

"All of the equations that were given in the book, there was just a lot of information thrown out all at once, but I should be able to understand it after I have to use it for a problem."

"Pretty much this whole chapter was confusing. I didn't take as much time reading it as the other chapters though, so that could play a part in that."

"I found it confusing knowing which law to apply in terms of Newton's first law or Newton's third law, but it was better explained in class and I understand now."

What is the meaning of the "normal" in the "normal force?"

"'Normal' means perpendicular to the surface."

"The 'normal' comes from the fact that it is always there when an object is in contact against a surface?"

"Any forces already acting on the object of concern?"

"'Normal' force usually refers to forces that act naturally in a sense?"

"Does it mean 'instantaneous?'"

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

"The coefficients are unitless."

"Trick question, there are none."

"I am truly confused."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [83%]

An external horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [68%]

An external horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [71%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied on it, so it slows down:
f_k = µ_k·N. [22%]

An external horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [17%]

An external horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [39%]

An external horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [37%]

An external horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [22%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I struggled with these questions and probably need some more explanation on them."

"I don't understand how to determine which forces cancel out. Also, if the net force is negative, what would happen?" (Determining which forces are exerted on an object depends on the wording of the problem. Deciding whether these forces cancel or not depends on whether the motion is constant or changing. If a net force is negative, then it points in the negative direction (depending on how you defined your positive direction); but usually we denote magnitudes of forces (keeping them all positive), and explicitly denoting their direction in words (left/right/up/down) or arrows.)

"When would f_k = some value between 0 and µ_k·N? Is that possible?" (That would never be possible. The kinetic friction force (assuming that the object is already unstuck, and has motion (constant or changing) would always be equal to µ_k·N.)

"Could you please go over what the values of static and kinetic frictional forces mean? I'm having a hard time grasping the concept of when kinetic friction force equals zero, and so on." ("Kinetic" means "in motion," so if the object is still stuck to a surface, then kinetic friction force is zero.)

"I could not find the presentation preview." (There were no presentation slides for this topic; just the textbook chapters.)

"You are really helpful going over the chapters and material in class. It helps me out a lot." (You're helping me out a lot by telling me what you specifically understand or are confused about when I prepare for each class.)

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"The coefficient of static friction is a ratio of the maximum static frictional force over the normal force. Therefore in order to find the maximum static frictional force you must multiply the coefficient of static friction by the normal force. The equation is the same when finding the kinetic frictional force just using the coefficient of kinetic friction."

"Static frictional force is the force required for an object to move from rest and is always greater than kinetic friction. Kinetic friction opposes an object's sliding motion on a surface while it is in motion. Newton's first law applies to objects at rest on a surface until it is pushed or pulled enough past the static frictional force then Newton's second law applies."

"Friction is caused by the molecular bonds between two surfaces. There is kinetic friction which is when the force is slowing down to a stop and static friction which is when the force is unable to move. The definitions are easy to understand, but I think applying them to the problems is what is most difficult."

"The two types of friction are static and kinetic. From what I understand, static friction is the force that keeps an object at rest and must be overcome for an object to move along another. On the other hand, kinetic force is the force that slows an object down once in motion."

"I have a fairly strong grasp on how Newton's second law applies to cases with a non-zero net force, and changing motion."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I was having difficulty with Newton's first law and third law because they seem similar. However, going over different practice problems I feel a little more confident."

"Kinetic friction--I thought I understood it but answering the questions below about the values of f_k in different situations left me more confused."

"I'm honestly still getting confused between normal force and net force. I've tried looking at some examples, but singling these two out from each other messes me up."

"It is hard to visualize forces especially if there is a movement. But it will help to learn about free-body diagram to clarify those situations."

"How to apply these concepts of the frictional forces when also incorporating what we have learned with normal forces, weight, etc."

"When kinetic force is in play with normal force and weight force. It's apparent that they're acting in different ways on a object and I can separate them in my head. Figuring out when one is at play or when one affects another is still eluding me."

"I think I understand the concepts well, but I am confused as to the application of the two equations in determining behavior of an object under static or kinetic force."

"Most of the material was understandable."

"Not much was confusing."

What is the meaning of the "normal" in the "normal force?"

"'Normal' means 'perpendicular.'"

"Normal force means the force perpendicular to the object; 'normal' and 'perpendicular' tend to be interchangeable."

"The force that objects in contact exert on each other perpendicular to their planes. I don't know that I can clearly convey the 'normal' part. I just think of it as 'normal' because the objects are just sitting there, exerting this force as long as a they are in contact."

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

"Newtons?"

"I have no idea."

"I'm confused."

"Both friction coefficients are dimensionless."

"They both have no units."

"I am a little confused by this. I thought there were no units, but I may have misunderstood."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [74%]

An external horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [72%]

An external horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [64%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied on it, so it slows down:
f_k = µ_k·N. [27%]

An external horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [20%]

An external horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [41%]

An external horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [48%]

An external horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [27%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Need some clarification on this, mostly on kinetic friction."

"I'm gonna need some review of this in lecture format please!! :("

"I kind of find putting all the laws and everything together a bit confusing. It just seems there's not much lecture in class."

"I think the problem I have is applying these concepts to problem solving. I believe it's just gonna take a lot of practice so please continue with the examples in class."

"Kinetic friction examples would be awesome."

"Newton's third law is used with a force between two interacting objects. Newton's first law is for objects at rest (or constant velocity). I would like to see more Newton's second law examples."

"I would appreciate overview in class of concepts rather than just worksheets on them. I know you say that it is proven that lectures aren't efficient but maybe you could meet students in the middle with some brief lecturing." (I can't really meet everyone in the middle, because everyone needs different things. At least with getting feedback from these reading assignments I can make an informed decision on what/how to cover things in class, whether it is by lecturing (especially today on how static and kinetic friction behave differently), worksheets (so you can get immediate feedback on applying what you just learned), and/or worked-out examples (by me on the whiteboards, or turned in by you at the end of class). But if you are still not getting what you specifically and individually need during class time, then you need to be proactive and ask me question in person or by e-mail, come to my office hours, or go to the tutoring center.)

"If something slides across a smooth surface with some kinetic friction and then continues onto a rougher surface, how does this change the kinetic friction?" (Different materials, different μ_k coefficients; so the amount of kinetic friction force will change.)

"What is something that can slow an object down but not be considered kinetic friction?" (Drag (air resistance)? Like when a parachute is deployed?)

"These questions seem like trick questions but I'm not sure." (They're meant to make you think. Or trick you into making you think.)

"What instrument can measure the friction coefficient?" (Not directly, but you can measure the friction force by pulling on it with a force sensor, and know the mass of the object (so you can calculate its weight, which is equal to the normal force by Newton's first law), then you can set your experimental friction force value equal to μ_s·N, and solve for the coefficient. Which is pretty much what you'll be doing in lab next week.)

"Physics is interesting but also very confusing."

"Does Newton's third law apply to static friction? Say an object is not moving yet even when an external applied the force is acting on it to try to make it move. So is there a reaction force that exerts a resisting opposite reaction force?" (Hold on, we're going to need to dig pretty deep to answer this question. This stationary object will have two horizontal forces acting on it: an external applied force (say, you pushing it to the right), and the static friction force that opposes the attempted unsticking (which would make it point to the left). Since the object is still stationary, these two forces are opposite in direction and equal and magnitude, but they would not be related via Newton's third law (as they are two different types of forces: the "applied force" would be a normal contact force (if you are pushing on the side of the object), and the static friction force would be a, well, friction contact force. However, the applied normal contact force of you on the side of the object (directed to the right) would be the third law pair of the normal contact force of the side of the object exerted on you (directed to the left). Also the static friction force of the floor on the object (directed to the left) would be the third law pair of the static friction force of the object on the floor (directed to the right). You should verify that all these third law pairs satisfy the "POF-OST-ITO" checklist, while the first law pair mentioned earlier does not.)

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"I understand what the static friction force is. Also when a object is stationary on a level surface, the force applied must be greater than the maximum static friction force in order for the object to move."

"Static friction between two surfaces opposes (attempted) motion. Kinetic friction opposes sliding motion once the object is moving."

"Friction is bad and makes physics much harder to calculate."

"Friction is a force that prevents things from moving (static) or shows something down (kinetic). It always apposes movement and is parallel to the surface."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"When to apply each of Newtons three laws."

"I don't understand everything about physics but that's okay, that's why I'm taking this class."

"I found the equations for static and kinetic friction to be a little confusing, as well as what we use their coefficients for."

"I still find free body diagrams really confusing. I definitely need to work on drawing them out and really understand all of the parts that I am drawing."

"I found the force vectors to be confusing. I get the different type of forces mixed up."

What is the meaning of the "normal" in the "normal force?"

"The 'normal' in normal force means that the force is perpendicular to the two objects in contact."

"It is the support exerted when an object is in contact with a stable surface."

"'Normal' means 'perpendicular.'"

"Something about triangles?"

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

"These coefficients have no units. They depend on the type and condition of the two surfaces that are in contact."

"Newtons?"

"Microseconds?"

"The units are in N or newtons?"

"psi?"

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [83%]

An external horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [69%]

An external horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [69%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No external horizontal forces applied on it, so it slows down:
f_k = µ_k·N. [19%]

An external horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [14%]

An external horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [58%]

An external horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [44%]

An external horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [22%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Where can I get extra help?" (Our embedded tutor has help sessions at the Learning Resource Center; these hours are posted on the course website. I also have office hours, too.)

"When an object attempts to move we are concerned with static friction. As soon as the applied force exceeds the maximum static frictional force does static friction become irrelevant and we become concerned with kinetic friction?" (Yes.)

"I didn't quite understand how the units and coefficients of friction go hand in hand, if it's a coefficient doesn't that mean it's just a number?" (Yes, it's like a "tackiness" ratio for two given surfaces.)

"I get static friction, but what I really don't understand is if kinetic friction should equal zero, or whether it should equal μ_k times the normal force." (Kinetic friction always equals μ_k times the normal force. Always.)

"Could you go over more of the kinetic friction and static friction and how to know if it is zero, between zero and its maximum value, or at its maximum value?"

"I did not understand the kinetic friction force questions...at all."

"Could we talk more about static friction and kinetic friction please?"

"This is all just confusing."

"I am not sure when to use each law specifically. I guess what I understand most are Newton's first law, and the second law. I understand the difference between the two, but once you throw the third law in there, it gets confusing."

Physics quiz question: forces pushing on still-stuck suitcase, crate

Physics 205A Quiz 3, fall semester 2015
Cuesta College, San Luis Obispo, CA


A horizontal force of 45 N is applied to the left on a suitcase, and another horizontal force of 45 N is applied to the left on a crate. The suitcase is more massive than the crate. Both the suitcase and the crate are motionless, and both have the same static friction coefficient µ_s with the horizontal floor.

The __________ has a greater magnitude net force exerted on it.
(A) suitcase.
(B) crate.
(C) (There is a tie.)
(D) (Not enough information given.)

Correct answer (highlight to unhide): (C)

Since the suitcase is stationary, its motion is constant (with zero velocity). Thus the net force of all the forces acting on the suitcase must equal zero, due to Newton's first law. Similarly for the crate, which is also stationary, and thus the net force of all the forces acting on the crate must also equal zero, due to Newton's first law.

Sections 70854, 70855, 73320
Exam code: quiz03re3T
(A) : 15 students
(B) : 7 students
(C) : 51 students
(D) : 1 student

Success level: 70%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.48

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2015
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"Static friction acts to prevent objects from starting to slide. Kinetic friction acts to try to make sliding objects stop sliding."

"Kinetic friction occurs when two objects are moving relative to each other while static friction is when the two objects are not moving. Kinetic friction is usually less than the static friction."

"Different materials have different coefficients of friction."

"Frictional forces are parallel to surfaces."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"Still having difficulty distinguishing the differences of Newton's first law and the third law."

"The differences between static friction and kinetic friction. Not sure how they are different."

"I don't really understand how to identify the magnitudes of static and kinetic frictional forces, and how to tell the difference(s) between them."

"I got this."

"I would like to go over more problems and more thorough explanation of the concepts during lecture."

What is the meaning of the "normal" in the "normal force?"

"Perpendicular."

"The perpendicular component of the force that acts on the object from a surface."

"'Normal' force is the average force exerted."

"It's the force 'normally' existing because of gravity?"

"The force that is applied due to gravity."

"I'm not sure."

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

"These are measured in newtons (N)."

"Micros."

"These coefficients are unitless."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [78%]

A horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [75%]

A horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [73%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No horizontal forces on it, so it slows down:
f_k = µ_k·N. [20%]

A horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [14%]

A horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [58%]

A horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [47%]

A horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [29%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I don't understand static friction vs. kinetic friction--the book doesn't do a great job at explaining each and whether their magnitude is zero, or not zero, etc."

"I like your online presentation previews much more than only reading from the text."

"We're going to need to talk about this." (Yes. A lot.)

"Like that you lectured last time--I actually learned a little. Can you please keep doing it that way? But instead teach us everything that will be in the midterms :D" (Hopefully maybe such that you can answer essay questions on friction on the midterm. #dreams #squadgoals)

"I LOVE PHYSICS." (Yes, but does physics love you back? #noitdont)

Online reading assignment: applications of Newton's laws (friction)

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws (emphasizing static and kinetic friction).


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"A contact force perpendicular to the contact surface that prevents two objects from passing through one another is called a normal force. I also understand that a normal force only acts upon the contact surfaces and does not take other forces into account."

"Friction is caused by atomic or molecular bonds between the high points on the surfaces of two objects. We can never assume anything about the magnitude of the normal force."

"Kinetic friction is a force that seeks to stop motion. Static friction is a force to prevent movement from starting."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"The formulas! I still don't understand how to solve for these forces, or what all the different symbols mean."

"Separating Newtons three laws from each other. Knowing which law to apply to which situation."

"Some confusion over kinetic and static friction."

"While I understand these abstract concepts I am not confident in being able to apply all of them to problem solving. I look forward to going through problems in class to clear up misunderstandings I don't know I have and practice using this knowledge."

What is the meaning of the "normal" in the "normal force?"

"I have no idea."

"A weight or counter force to resist the weight."

"In geometry 'normal' means 'perpendicular.' Therefore normal force is always perpendicular to the contact surface."

The SI (Système International) units of the static friction coefficient µ_s and the kinetic friction coefficient µ_k are:

"Huh?"

"Newtons."

"They are dimensionless, as the units of the magnitudes of forces divided by each other cancel out."

"There are no units--the coefficients represent proportions (between the friction forces and normal force)."

Identify the magnitude of the static friction force f_s for each of the following situations of a box that is initially stationary on a horizontal floor. (Only correct responses shown.)

No horizontal forces applied to it, so it remains stuck to the floor:
f_s = 0. [60%]

A horizontal force applied to it, but still remains stuck to the floor:
f_s = some value between 0 and µ_s·N. [68%]

A horizontal force applied to it, at the threshold of nearly becoming unstuck:
f_s = µ_s·N. [54%]

Identify the magnitude of the kinetic friction force f_k for each of the following situations of a box that is already sliding across a horizontal floor. (Only correct responses shown.)

No horizontal forces on it, so it slows down:
f_k = µ_k·N. [19%]

A horizontal force applied in the forward direction, but not enough to keep the box going so it still gradually slows down:
f_k = µ_k·N. [17%]

A horizontal force applied in the forward direction, just enough to keep the box going at a constant speed:
f_k = µ_k·N. [41%]

A horizontal force applied in the forward direction, enough to gradually increase the speed of the box:
f_k = µ_k·N. [41%]

A horizontal force applied in the backwards direction, such that the box slows down:
f_k = µ_k·N. [21%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Where can we find the answers to all the quiz flashcard questions? I keep doing them but am not sure I am doing them right." (Come in to office hours or e-mail me your answers to flashcard questions not already discussed in class, so I can "grade" them and give you feedback on the ones you missed.)

"Can we talk about these homework questions in class, please? Can you do more explaining just in general?" (Yes, at the start of each class we'll go over the online reading assignments questions that the class messed up on most, before we move to working on example problems and flashcard questions. But I need the feedback on the reading assignments on what you specifically understand or don't understand in order to make the best use of time.)

"Since kinetic friction is basically proportional to the normal force and is independent of speed, and the object sliding and the surface the object is sliding on remain the same, isn't the kinetic friction force always f_k = µ_k·N?" (Yes--always! Good job.)

"In-class examples showing actual values being used for µ_s or µ_k would be good, just to have an idea of generally what these values look like." (Yes, as time allows, we'll go over some examples from previous quizzes and exams.)

"Once we obtain all numerical values for all the different types of forces, what equations do we use?" (The process of finding the numerical values for the forces is the point of using Newton's laws. Forcewise, there isn't much else to do afterwards, but if you have the acceleration from Newton's second law, then you can always calculate velocity and position from that using the kinematic equations.)

Physics final exam problem: frictionless box pushing box with sliding friction

Physics 205A Final Exam, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.63

A 7.0 kg box and a 3.0 kg box are in contact with each other on a horizontal floor. The 7.0 kg box slides without friction on the floor, but the 3.0 kg box has a kinetic friction coefficient of 0.15 with the floor. A horizontal force of 5.0 N is applied to the 7.0 kg box such that both boxes are sliding together to the right. Determine the magnitude of the acceleration of these boxes. Show your work and explain your reasoning.

Solution and grading rubric:

• p:
  Correct. Draws free-body diagrams, and applies properties of forces and Newton's laws to determine the kinetic friction force on the 3.0 kg box, finds the net horizontal force on the system of both boxes, and implements Newton's second law to find the acceleration of both boxes.
• r:
  Nearly correct, but includes minor math errors. One of the two points (1)-(2) correct, other is problematic/incomplete.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. At least some attempt at applying properties of forces and Newton's laws.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach does not substantively use properties of forces and Newton's laws.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855, 73320
Exam code: finaln0M3
p: 13 students
r: 4 students
t: 7 students
v: 17 students
x: 13 students
y: 2 students
z: 5 students

A sample "p" response (from student 0000):

Physics midterm question: net force on pushed-down book

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.37

A force of 1.0 N pushes down on a 0.50 kg book, which is stationary on a table. The direction of the net force on the book is:
(A) downwards.
(B) (has no direction, as the net force is zero.)
(C) upwards.
(D) (Not enough information given.)

Correct answer (highlight to unhide): (B)

Since the book is stationary, its motion is constant (with zero velocity). Thus the net force of all the forces acting on the book must equal zero, due to Newton's first law.

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 12 students
(B) : 57 students
(C) : 4 students
(D) : 0 students

Success level: 78%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.24

Physics midterm question: forces on pushed-down book

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.37

A force of 1.0 N pushes down on a 0.50 kg book, which is stationary on a table. Newton's __________ law tells you that these two forces are equal and opposite in direction:

The applied force pushing on the book.
Weight force of the book.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)
(E) (Not enough information is given.)

Correct answer (highlight to unhide): (D)

The book has three vertical forces acting on it:

Weight force of Earth on book (downwards, magnitude w = m·g = 4.9 N).
Normal force of table on book (upwards, magnitude N = ?).
Applied force pushing on book (downwards, magnitude F_applied = 1.0 N).

Because the book is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two downwards forces (weight and applied force) are together equal to the one upwards force (normal), such that the normal force must have a magnitude of 5.9 N. Thus the two forces listed above (applied force and weight force) are neither equal in magnitude (1.0 N and 4.9 N, respectively) nor opposite in direction (both are downwards).

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 21 students
(B) : 3 students
(C) : 8 students
(D) : 41 students
(E) : 0 students

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66

Physics midterm question: accelerating/decelerating upwards moving elevators

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.23(a)

Two elevators have the same mass of 800 kg, and both move upwards pulled by cables. Elevator 1 moves with an increasing upwards speed, while elevator 2 moves with an decreasing upwards speed. Discuss why the tension in the cable pulling elevator 1 is greater than the tension in the cable pulling elevator 2. Explain your reasoning by using the properties of forces and Newton's laws.

Solution and grading rubric:

• p:
  Correct. For both elevators, a cable exerts an upwards tension force, while Earth exerts a downwards weight force. The downwards weight force has the same magnitude ((800 kg)⋅(9.80 N/kg) = 7.8×10³ N) for both elevators. Discusses that the acceleration of elevator 1 must point upwards, and so must the net force on elevator 1, and thus the tension force on elevator 1 must be greater than the weight force of 7.8×10³ N; and the acceleration of elevator 2 must downwards, and so must the net force on elevator 2, and thus the tension force on elevator 2 must be less than the weight force of 7.8×10³ N.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May have net forces pointing upwards for both elevators; weight force vectors drawn with different lengths for the elevators; tension force on elevator 2 more than the weight force (but less than the tension force on elevator 1), etc.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least some application of Newton's laws in relating direction net force with acceleration, and how two opposite forces combine to obtain a net force.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some attempt at drawing a free-body diagram and discussing properties of forces.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Application of concepts other than those of forces and Newton's laws.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01p0To
p: 11 students
r: 16 students
t: 13 students
v: 32 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 1216):

Physics midterm problem: range of possible static friction coefficient values

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.63

A force of 10.0 N pushes on 5.0 kg crate that is initially stationary, and as a result it becomes unstuck and begins to slide. When the 2.0 kg book is stacked on top of the crate, a force of 10.0 N pushes on the crate, and both book and crate remain motionless. Determine a plausible numerical value for the coefficient of static friction µ_s for the crate and the floor. Show your work and explain your reasoning using a free-body diagram, the properties of forces, and Newton's laws.

Solution and grading rubric:

• p:
  Correct. Draws free-body diagrams and applies Newton's laws and definitions of maximum static friction forces. For the crate, the applied force of 10.0 N must be just at or above the maximum static friction force of µ_s⋅(49 N), which yields a maximum value of 0.20 for µ_s. For the book stacked on the crate, the applied force of 10.0 N must be below the maximum static friction force of µ_s⋅(68 N), which yields a minimum value of 0.15 for µ_s. Thus the static coefficient of friction µ_s between the crater and floor must have some specific value between 0.15 and 0.20. May instead have determined that µ_s = 0.20 for the critical case of the applied force just being able to unstick the crate, and demonstrates that this µ_s value would result in a maximum static friction force of 14 N for the book on crate, such that it would remain stationary; or determined that µ_s = 0.15 for the critical case of the applied force just being able to unstick the book on crate, and demonstrates that this µ_s value would result in a maximum static friction force of 7.4 N for the crate, such that it would become unstuck.
• r:
  Nearly correct, but includes minor math errors. Determines bounding values for µ_s (0.15, 0.20), but does not explicitly interpret µ_s must be some value between 0.15 and 0.20.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Has free-body diagram and methodical application of Newton's laws to determine one of the boundary values of µ_s from one of the two cases, but does not explicitly demonstrate that it would apply to the other case.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Free-body diagram identifies most forces and their directions, with some attempt at applying Newton's laws.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Garbled/incomplete free-body diagram with little to no application of Newton's laws, etc.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01p0To
p: 18 students
r: 9 students
t: 9 students
v: 33 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 0825), finding the range of possible µ_s values:

Another sample "p" response (from student 0494), demonstrating that the lowest possible µ_s value would work in both cases:

Physics quiz question: pulling on still-stuck crate

Physics 205A Quiz 3, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.53

A rope exerts a tension force of 4.0 N on a 5.0 kg crate, which remains motionless on a non-frictionless floor. The magnitude of the static friction force on the crate is __________ 4.0 N.
(A) less than.
(B) more than.
(C) exactly equal to.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

The crate has two vertical forces acting on it:

Weight force of Earth on crate (downwards, magnitude w = m·g = 49 N).
Normal force of floor on crate (upwards, magnitude N = 49 N).

Because the crate is stationary in the vertical direction, these two forces are equal in magnitude and opposite in direction, due to Newton's first law.

The crate has two horizontal forces acting on it (as it moves to the right):

Static friction force of floor on crate (to the left).
Tension force on crate (to the right, magnitude T = 4.0 N).

Since the crate remains stationary, Newton's first law is applicable here, and thus the static friction force and applied tension force are equal in magnitude and opposite in direction. (Note that the maximum amount of static friction force that must be overcome in order for the crate to become unstuck should be some value greater than 4.0 N.)

Sections 70854, 70855, 73320
Exam code: quiz03eL3v
(A) : 9 students
(B) : 43 students
(C) : 17 students
(D) : 1 student

Success level: 24%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.11

Online reading assignment: applications of Newton's laws

Physics 205A, fall semester 2013
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters on applications of Newton's laws.

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.

"The problems for Newton's laws were a lot more conceptual than the projectile motion problems. You have to think about what forces act on each other and be very familiar with Newton's three laws."

"Friction force is interesting because it is almost always present, and when I used to play soccer, I would always consider how the grass would affect how far the ball rolled when it was thick compared to thin and short."

"That there are high points on surfaces that are invisible to our eyes, but are the cause of friction forces. The smoother the surface the less resistance."

"Honestly! If you want better traction in a rear-wheel drive car and the road, put something heavy in the trunk to increase normal force. I like real world/real situation examples."

"That when two objects are interacting with each other they have two forces working on them, which is the same force just looked at from opposite directions."

"I enjoyed learning about friction (still don't feel I have a full grasp on it). Learning about forces has really changed my perspective on terms such as weight and pull/push."

"The rules for friction were interesting. It was a challenge to do the free-body diagrams, but once I got the hang of them it was interesting to see the applications."

"That there are two different types of friction forces."

"Friction is caused by bonds between atoms that form between the 'high points' of the two surfaces that come in contact."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"I think I understand Newton's laws to a certain extent, but I still am not very confident in applying them to a variety of situations."

"I just lack confidence in drawing free-body diagrams and real-world applications because it is hard for me to put the formal information into a picture."

"The differentiation between sliding friction and static friction. I don't understand how static friction can become sliding friction if the friction does not prohibit sliding."

What is the meaning of the "normal" in the "normal force?"

"In geometry, 'normal' means 'perpendicular.'"

"'Normal' in 'normal force' merely means the force pushed on from one object resting on another. Its the 'regular' force that is pressed on one object to another."

The magnitude of the static friction force on an object that is being pushed, but is still stationary:

is zero.	***************** [17]
is a constant, non-zero value.	********** [10]
could have any value up to a maximum amount.	************************** [26]
(Unsure/guessing/lost/help!)	******* [7]

The magnitude of the kinetic friction force on an object sliding at constant speed:

is zero.	**** [4]
is a constant, non-zero value.	**************************************** [40]
could have any value up to a maximum amount.	****** [6]
(Unsure/guessing/lost/help!)	********** [10]

The magnitude of the kinetic friction force on a sliding object that is accelerating:

is zero.	** [2]
is a constant, non-zero value.	********************* [21]
could have any value up to a maximum amount.	************************ [24]
(Unsure/guessing/lost/help!)	************* [13]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Why are we learning this?" (Because this is a physics class.)

"Can you go over a homework problem in every class if we are not getting it, and to not just say that the answer is in the back of the book. It was really helpful when you did that last class session." (If the majority of the class reports having issues with that problem, and if time allows during the brief lecture period in class. Otherwise there will be time to individually go over homework problems during the group whiteboard problem-solving period at the end of every class.)

"In class last week you tried to explain weight force versus normal force. Can you go over that again?" (Weight force is the gravitational force of Earth pulling down on an object; it always has the magnitude w = m·g. normal force is a contact force exerted by a supporting surface; its magnitude can have any value ranging from 0 (surfaces not actually touching), any positive value (where the surfaces are being pressed into each other), up to a maximum value (where the surfaces can no longer support any greater force, and would begin breaking).)

"Can we go over problems relating to applying Newton's second law? Can we please go over the word problems as a class? I'd like to spend some time going over the examples." (Yes, yes, and as time allows.)

Physics final exam problem: weights added to boxes about to slide

Physics 205A Final Exam, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 4.74, 4.85

A 3.0 kg box on a horizontal table is attached by a string to a hanging 1.2 kg mass. The string and pulley are ideal, but the table is not frictionless. A Physics 205A student observes that the 1.2 kg hanging mass is just sufficient enough to overcome static friction between the 3.0 kg box and the table. The student resets this experiment, adding 0.1 kg to the box, and 0.1 kg to the hanging mass. Determine whether static friction between the box (now 3.1 kg) and the table would be overcome by the hanging mass (now 1.3 kg). Show your work and explain your reasoning using a free-body diagram, the properties of forces, and Newton's laws.

Solution and grading rubric:

• p:
  Correct. Draws free-body diagrams, and applies properties of forces and Newton's laws to determine the static friction coefficient between the box and table for the first case; then for the second case determines that static friction would be overcome for the box either by discussing how (a) the hanging mass will exert a tension force on the box greater than the maximum static friction force on the box, or (b) the required static friction coefficient between the box and table to remain stationary is greater than the static friction coefficient in the first case, and since the static friction coefficient remains constant, the static friction on the box will again be overcome.
• r:
  Nearly correct, but includes minor math errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. At least some attempt at applying properties of forces and Newton's laws.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach does not substantively use properties of forces and Newton's laws.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: finalPr0p
p: 16 students
r: 3 students
t: 8 students
v: 11 students
x: 9 students
y: 2 students
z: 2 students

A sample "p" response (from student 4585):
Another sample "p" response (from student 7582):