## 20140917

### Online reading assignment: Newton's third law

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 and previewing a presentation on Newton's third law.

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.
"When two objects interact they both exert a force on the other."

"I have a great understanding of Newton's first and second laws using the motion flow chart. It makes the questions much easier."

"Whereas Newton's first and second laws have to do with motion and net force, Newton's third law has to do with symmetry."

"Interaction pairs are formed when two objects are in contact with one another and exert equal, yet opposite, forces on each other. Not all equal/opposite forces are an interaction pair, as there must be two objects interacting with each other."

"Newton's third law says that interaction partners always have the same magnitude and are opposite in direction. Also, Newton's third law isn't involved every time two forces happen to be equal and opposite."

"One of the differences between Newton's first law and Newton's third law is that Newton's third law requires the same type of forces (weight-weight, etc.), whereas Newton's first law can involve two different types of forces."

"I understand the three-part checklist to determine if Newton's third law applies. If even one check fails, Newton's third law does not apply."

"One can describe the same force in two different ways because of Newton's third law."

"I understand that I need to do this reading!"

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 am confused by what it is talking about in 'a system.' I do not understand what it is distinguishing between an internal and external system."

"Newton's third law--specifically by in an interaction between two objects, whether/how each object's force is equal in magnitude to the other."

"I don't know how to distinguish between when Newton's first law applies and when Newton's third law applies. Also I wasn't 100% sure of the questions at the end of this form."

Consider two categories of motion: (1) Velocity that is constant and unchanging. (2) Velocity that is changing. Discuss whether or not there would be a third category of motion not already covered under these two categories.
"Something with no velocity?"

"How about velocity that is neither constant nor changing?"

"I am completely lost on this question and need to go over it in class."

"No, because either the acceleration is zero, or the acceleration is non-zero, which covers both categories."

"There would not be a third category--either velocity is constant or velocity is changing--it's a binary concept."

An "interaction pair" (or "interaction partners") refers to a pair of:
 objects. **** [4] forces. ************************************ [36] (Both of the above choices.) ***************** [17] (Unsure/guessing/lost/help!) [0]

According to Newton's first law, the normal force of the person's head on the stack of books is equal in magnitude and opposite in direction to the:
 normal force of the stack of books on the person's head. *************** [15] weight force of Earth on the stack of books. ************************ [24] (Both of the above choices.) ********* [9] (Neither of the above choices.) ****** [6] (Unsure/guessing/lost/help!) *** [3]

According to Newton's third law, the normal force of the person's head on the stack of books is equal in magnitude and opposite in direction to the:
 normal force of the stack of books on the person's head. *************************** [27] weight force of Earth on the stack of books. **************** [16] (Both of the above choices.) ****** [6] (Neither of the above choices.) **** [4] (Unsure/guessing/lost/help!) **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If the net force is not zero then is the velocity not constant anymore? Do we use Newton's second law of motion? (Yes, and yes!)

"In any case involving two (interacting) objects, are there always at least two of Newton's laws in effect? For example, Newton's first law and third law, or Newton's second law and third law?" (Yes--either the first law or the second law in terms of the motion of the objects (constant or changing), and the third law in terms of how the objects interact with each other.)

"Are internal forces meant to be inside or something? Or is it more like forces within a set of objects, and we call them a 'system' because we want to look at the forces that they are contributing as a whole?" (Yes...and yes.)

"I am not sure how to differentiate between Newton's first law and Newton's third law for forces acting on/between the stack of books and the person's head. I also don't really understand the homework questions." (I'm going to allocate time to carefully go over these at the start of the next class.)

"It would be more helpful to have class discussions instead of working in groups and then going over the correct answers." (Sure, as long as you keep asking good questions for me to answer, either here on the reading assignments, or during class. That said, committing your answers in writing and then having a high-level discussion verifying how you got those answers with your peers is an important part of the learning process emphasized in this class.)