## 20180917

### 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 µs and the kinetic friction coefficient µk are:
"The coefficients are unitless."

"Trick question, there are none."

"I am truly confused."

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. [83%]

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

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

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. [22%]

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. [17%]

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

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

An external horizontal force applied in the backwards direction, such that the box slows down:
fk = µk·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 fk = some value between 0 and µk·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 µk·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.)