## 20140922

### 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 µs and the kinetic friction coefficient µk 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 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. [60%]

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

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

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. [19%]

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. [17%]

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

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

A horizontal force applied in the backwards direction, such that the box slows down:
fk = µk·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 fk = µk·N?" (Yes--always! Good job.)

"In-class examples showing actual values being used for µs or µk 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.)