## 20160914

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

Physics 205A, fall semester 2016
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.
"That there is a method to determine if Newton's third law is in effect."

"It makes sense to me that all forces are interactions between two objects and that each object produces forces against each other. POF-OST-ITO is an easy way to remember the three questions that have to do with the Newton's third law and interpreting if the two forces are a third law pair."

"The POF-OST-ITO checklist in determining if Newton's third law is valid. I understand the difference between the third law in regards to the first two laws. The third law is more of a take on symmetry of the two objects."

"The idea that Newtons third law has nothing to do with changing motion or non-changing motion, but that it is rather a general idea about forces."

"All forces in the universe occur in equal but oppositely directed pairs. There are no isolated forces; for every external force that acts on an object there is a force of equal magnitude but opposite direction which acts back on the object which exerted that external force."

"This section was on Newtons third law that does not have anything to do with motion or net forces. Forces are interactions between two objects. We use a three part check list to see what two forces are related using Newtons third law. POF-OST-ITO has to work for all three parts in order for the third law to apply."

"I understood how opposite forces if they have the same magnitude will equal zero. I understand though if there are two forces going in different directions with different magnitudes then the forces will not be zero and Newton's second law will be applied."

"There are many different types of forces that have an impact on whether/how an object moves. The gravitational force helps to make sure that items do not just float away. The normal force and static friction force explains how a table can hold or prevent objects from moving; kinetic friction force explains how an object in movement can slow down when it is sliding across a table. Tension force--well let's be grateful for that so that the elevator doesn't drop us to a sudden death."

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.
"Only thing I found confusing was which forces to focus on when trying to decipher what law to use. Since we just started discussing the first two Newton's laws, understanding the third one is still sinking in, since I understand the other two more now."

"I didn't understand what the questions at the end of the blog were trying to get at (this is why I replied 'guessing' on them below). So, it would be helpful to go over those. Also, going over examples of how to work the problems would be great because the concepts seem to be fairly easy, but the problems were kind of confusing."

"I found it confusing when it said that because an interaction involves two objects, there will be two slightly different, but equally valid descriptions of the same force. If they both have the same equal force acting on an object, how are they slightly different descriptions of the force?"

"I'm having a little trouble understanding the normal force and I'm hoping that in class tomorrow I will have a better understanding of it."

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.
From my understanding the two categories are the only ones needed. So whether the velocity is constantly zero or constantly at a number other than zero, it is still constant. If the motion is changing, then that would fit the second category."

"From my understanding I believe that changing and constant pretty much covers all types of motion. I tried to think deeper. I'm not sure if I am overthinking or underthinking it."

"No, unless if the velocity doesn't exist, however that would mean it's stationary. So I think it's covered."

"I have no idea."

"Acceleration?"

"Maybe a velocity that is not at all? A velocity of zero?"

"Velocity that is...constantly changing? That sounds like a thing."

"I believe that there should be a third category should be that the velocity is unchanging?"

"Would an absence of velocity count as a third category?"

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. *********** [11] weight force of Earth on the stack of books. ************ [12] (Both of the above choices.) ********* [9] (Neither of the above choices.) *** [3] (Unsure/guessing/lost/help!) ***** [5]

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. ****************** [18] weight force of Earth on the stack of books. ********** [10] (Both of the above choices.) ****** [6] (Neither of the above choices.) * [1] (Unsure/guessing/lost/help!) ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I'm looking forward to understanding this better."

"I find the examples you do in class extremely helpful!"

"Please show lots of examples of Newton's laws to separate each of them!"

"Will we have a flowchart to help us decide when to use the three laws?" (You already have flowcharts to determine whether to use Newton's first or second law depending on the object's motion or resultant net force, and a mnemonic to test whether Newton's third law is applicable to a given pair of forces.)

"Can we go over the third possible category of motion in class?" (No, because there is no third possible category of motion. Either motion can be constant...or motion is changing. What other kind of motion could there be? #constanging #changstant #mindblown)

"Can we go over the equation for apparent weight?" (No, because (1) we're not even there yet, and (2) I would much rather you understand the process of identifying forces, relating the net force to its motion, and applying Newton's laws to determine an object's "apparent" weight, instead of using a given equation (which you are not going to be given anyway on a quiz).)

"Do we get points knocked off these assignments for not answering any of the questions correctly/sufficiently?" (You get all the points whether you answer correctly or incorrectly, but you only get partial credit if you leave answers blank, or don't have a substantive or thoughtful reply.)