## 20180912

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

Physics 205A, fall semester 2018
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 seem to have a pretty good understanding of the first two types of Newtons Law. Also that all forces are the interactions of two objects."

"Weight and mass are two different concepts, and that weight will vary depending on the gravitational pull and mass is always constant."

"That if Newton's second law is to be applied, net force and acceleration must have the same direction. For example, if acceleration points up, then net force also has to point up. Also, that all the forces exerted on a object that has no movement (is stationary with no change in velocity) must equal to a net of zero."

"I have an absolute obsession with elevators, so reading about elevators in the book about how accelerating up or down effects the apparent weight was magnificent. I also learned that you will be weightless if the elevator is in a free fall. I also learned I should start weighing myself in the elevator. force equals mass times acceleration. Lastly mass does not change whether you are on the moon or on Earth, but your weight does. "

"I understand the concept of Newton's third law. If there are two objects acting upon each other with the same type of force, and their forces are equal and opposite then it is Newtons third law. I also like the POF-OST-ITO mnemonic because it will help me remember the law."

"Newton's third law has nothing to do with net force or motion. Instead, it has to do with interaction between two objects, of the same type, exerting a force on each other."

"Newton's third law only applies if an object has an interaction with another object, and that the interaction must of the same type between the two objects (eg. tension and tension). Also, there are different categories of interactions."

"Newton's third law has nothing to do with motion or net force, but is instead a symmetrical approach. The POF (pair of opposite forces), OST (of same type), and ITO (involving two objects) is a three-part checklist used to determine whether or not Newton's third law applies to the problem at hand, and that if just one of step of this cheklist is is invalid, then the law doesnt apply."

"The normal force--that an inanimate object such as a table can have a force upwards against something on top of it."

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 found the POF-OST-ITO mnemonic a little confusing because I kept forgetting what it stood for. I also am still a little confused on how it works. I also found the stacked book questions below confusing."

"'Remember, because an interaction involves two objects, there will be two slightly different, but equally valid descriptions of the same force.' I found this sentence from the presentation slides confusing, so I will need to check the textbook. "

"I still kind of find the third law confusing."

"I didn't understand the 'action-reaction' equation and how it applies to the law. "

"I'm still not sure how to distinguish what kind of force something is. I guess I just don't understand what a normal force is. "

"I found the normal force confusing. I don't understand how if a brick is on the ground, there is a normal force going up."

"Honestly, the most confusing thing for this reading was pronouncing 'POF-OST-ITO.'"

"There wasn't much I found super-confusing from this reading assignment."

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 third category, because either velocity is constant or velocity is changing."

"I don't think there would be a third category because velocity is either changing or not changing."

"These two categories seem to be describing Newton's first and second law of motion."

"No, because Newton's third law does not deal with motion or net force, but expresses an inherent property of force itself, without any regards to the resulting motion or net force."

"I do not know of another category of motion, but I assume there is since we are given this text box to fill out."

"I can't think of another category of motion."

"No--velocity is either going to be changing or not changing."

"Velocity that is both constant and changing?" 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!) ****** 

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Could you elaborate more on how do you tell the difference between Newton's first law versus the third law?"

"For the stacked books on head example, doesn't Newton's first law just explain why the books aren't moving?" (Yes. But since there are interactions between the objects in this system, then Newton's third law applies.)

"I found the questions at the end of the presentation preview a little confusing. If the book is resting on someone's head then there is no motion so how does it apply to Newton's laws? I must be overthinking something." (Because there is motion, then Newton's first law applies to the forces that contribute to the net force. And because there are two (or more) interacting objects in this system, then Newton's third law applies.)

"This might be a silly question, but is it possible for an object that is considered to be motionless/stationary and not have Newtons first law apply? Like say if a book is 'stationary' on a rotating turntable, would the net forces acting on the book be considered zero? Or does the book have to be completely stationary based on what object it is on." (Newton's laws apply only in inertial reference frames, where the "base" is either stationary, or moving with a constant velocity. Being on a rotating turntable is a non-inertial reference frame, so that requires a modified set of Newton's laws we're not going to consider in the class. So if an object is motionless in an inertial reference frame (which is itself motionless or has constant velocity), then Newton's first law applies.)

"Will we need to memorize the universal gravitational constant as well as Earth's mass and radius for gravitational force questions?" (No, because numerical values are usually given on quizzes and exams, and also the only gravitational force that we'll be using is just the weight force (the gravitational force due to Earth at sea level).)

"Looking forward to next class."