## 20190911

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

Physics 205A, fall semester 2019
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 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.
"I understand that there are five different types of forces (static, kinetic, weight, tension, and normal) as covered in the presentation. I also understand that Newton's first law requires an object to have a constant speed and direction."

"Newton's third law concept-wise is fairly straightforward, for whenever one object exerts force on a second object, the second object exerts an oppositely directed force of equal magnitude on the first object."

"What I understood from the assigned reading was POF-OST-ITO. POF stands for 'Pair of Opposite Forces.' OST stands for 'Of Same Type,' and ITO stands for 'Involving Two Objects.' If there are two forces in question satisfy all three parts of the POF-OST-ITO checklist, then Newton's third law applies."

"Newton's third law and how it has nothing to do with motion or net force but is a fundamental property of force itself. I also understand interaction pairs and how all forces are the interactions between two objects, and the POF-OST-ITO checklist."

"The POF-OST-ITO checklist and how it helps us determine if two forces are covered under Newton's third law."

"I understand, possibly, the most basic concept of this chapter--forces come in pairs. Basic but important, as it leads into the first question of the POF-OST-ITO checklist, 'Pair of Opposite Forces?' An object that receives some force by another object will exert a force back onto the other."

"I understood the differences in force types, (hopefully, assuming I got the multiple choices correct) and I found the normal force to be most interesting, as it is the force that stops stationary objects from continuously falling, counteracting the object's weight force. Even though it's a simple concept, it's interesting to have it explained in the world of physics, rather than through observation in the real world."

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.
"In all classes I've ever had, when I see the Σ summation symbol my mind just goes somewhere else. I understand it is some simple summation, but I'm sure seeing it with values will help."

"I think I understand the POF-OST-ITO checklist for Newton's third law, but I am not entirely sure. I would appreciate some review on that."

"Newton's third law and the POF-OST-ITO mnemonic kind of scares me. I understand that it's just a mnemonic used to confirm that Newton's third law applies, but it's still new and scary."

"The most confusing part was POF-OST-ITO checklist, and exactly how Newton's first law and Newton's third law differ. If any of the POF-OST-ITO checklists fail, is it automatically a Newton's first law scenario?"

"The mnemonic is a little difficult to remember. There is a kind of confusing relationship between Newton's first law and third law."

"I am having difficulty understanding the difference between Newton's third law and how it relates to the first two laws."

"Something I found confusing in our reading was the section about normal force, weight, and net force go altogether. I do not how to solve a problem like that."

"I found the statement 'the word tension is commonly used to mean the tendency of the rope to be pulled apart.' I found this confusing because I wasn't sure if tension then required two different forces pulling on an object from opposite ends."

"I do not understand tension very well. I do not know if it is based on mass or weight. Tension could also be an individual force, just not a fundamental force?"

"When it comes to solving tension problems, I start getting confused when there are angles involved. How do we use x- and y- components for tension forces?"

"I have not finished the reading but will do so as I will be caught up by the end of the week!"

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.
"NO other category of motion exists."

"Logically, and given the two choices, I would say no. (I have no idea if this is right, but here goes.) If the two categories are 'changing' and 'not changing,' I don't really there's an option for 'somewhere in the middle changing.'"

"I do not think there is another category of motion that would not be covered by these two categories. The only other option would be if an object is not in motion, however this would mean they have a (constant and unchanging) zero velocity."

"There would not be a third category because these two categories involve velocity and whether it's constant/unchanging or changing. These two categories fall under Newton's first and second laws, however his third law is more focused on equal and opposite/reactions based off force pairs, or forces in general." 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. ******************  weight force of Earth on the stack of books. ********************  (Both of the above choices.) **  (Neither of the above choices.) *****  (Unsure/guessing/lost/help!) 

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. **********************  weight force of Earth on the stack of books. ***********  (Both of the above choices.) ***********  (Neither of the above choices.) *  (Unsure/guessing/lost/help!) 

Identify the type of interaction ("force") with its symbol. (Only correct responses shown.)
Weight ("gravitational force") : w [91%]
Surface contact force ("normal force"): N (or FN) [91%]
Tension ("rope/cable/string force"): T [93%]
Kinetic friction ("sliding force," or "sliption"): fk [84%]
Static friction ("sticking force," or "stiction"): fs [89%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we think of the first law as using a single object as the reference point when determining forces and the third law as using two objects as reference points when determining how the forces of each act upon each other?" (Yes, exactly.)

"This was an interesting reading! I could use a little bit of clarification on a few things, but I'm sure I'll get it after the upcoming lecture."

"Can we have a quick clear summary of each force?" (We'll introduce them in-depth one at a time, in the examples we'll go over in class.)

"Please go slow for this particular section..... explain throughly."

"Will we have a flowchart for Newton's third law?" (You have the POF-OST-ITO checklist.)

"How much will we be doing universal law of gravitation problems?" (Actually, none. We're going to stay on this planet when it comes to doing physics.)

"Can you please clarify the difference between weight and mass." (We definitely will.)

"How would Newton's laws appear in a math problem?" (They're important concepts you should apply even before setting up a free-body diagram or solving an equation.)

"I appreciate your sternness when it comes to teaching these concepts, I thought you were gonna be goofy all semester."

"Will this be on the midterm?" (On both Quiz 3 and the first midterm, which is cumulative.)