20150921

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 µs and the kinetic friction coefficient µk are:
"These are measured in newtons (N)."

"Micros."

"These coefficients are unitless."

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 horizontal forces applied to it, so it remains stuck to the floor:
fs = 0. [78%]

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

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

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 horizontal forces on it, so it slows down:
fk = µk·N. [20%]

A 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%]

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

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

A horizontal force applied in the backwards direction, such that the box slows down:
fk = µk·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)

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