## 20170925

### 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 fk 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 µs and the kinetic friction coefficient µk 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 fs 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:
fs = 0. [74%]

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

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

Identify the magnitude of the kinetic friction force fk 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:
fk = µk·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:
fk = µk·N. [20%]

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

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

An external horizontal force applied in the backwards direction, such that the box slows down:
fk = µk·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 μk 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 μs·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.)