## 20160919

### 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.'"

The SI (Système International) units of the static friction coefficient µs and the kinetic friction coefficient µk 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 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. [69%]

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

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. [19%]

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. [14%]

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

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

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.
"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 μk times the normal force." (Kinetic friction always equals μk 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."