## 20171030

### Online reading assignment: rotational dynamics

Physics 205A, fall semester 2017
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 rotational dynamics.

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.
"Angular speed = angular displacement/time. The distance a car moves is equal to the arc length the tire rolls."

"How to calculate the KErot and all that. Its a little unclear but I think I understand that the most."

"When an object has both rotational kinetic energy and translational kinetic energy."

"Rotational kinetic energy is the energy of rotational motion. The faster an object rotates, the more KErot energy it has. Objects can have both rotational kinetic energy and translational kinetic energy."

"There is a lot of information. I understand the difference between translational and rotational speed, and the relationship between mass, and rotational inertia, and that all of these factors allow for different equations that calculate inertias for different objects. The transfer/balance equations are straightforward."

"I will be honest, I spent today studying for the quiz on Monday and am scared learning new material right now will interfere with the material I am trying to make sure I know for tomorrow. I did attempt the homework though."

"Honestly I didn't understand anything from the 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.
"Most of this is very confusing. A lot of new stuff at once is hard, especially when trying to study for a quiz that doesn't involve this new material."

"Most of this, actually."

"The difference between rotational kinetic energy and translational kinetic energy."

"I understand that rotational kinetic energy and translational kinetic energy are different. However, apart from that, I think I might need a better understanding on the differences between the two."

"I need more explanation on angular speed. And the gravitational potential energy, the translational kinetic energy, and the rotational kinetic energy."

"Rolling without slipping--I would appreciate going over this in class."

"KEtr and KErot is little confusing and it would be nice to go over it in class."

"Moments of inertia. I do not know where to begin for applications."

"I understand this section."

What is the SI (Système International) unit for rotational kinetic energy?
"kg·m2/s2."

"J, joules."

Describe an object that only has rotational kinetic energy, and no translational kinetic energy.
"Anemometer."

"Spin bike."

"Carousel ride."

"A record spinning."

"A spinning basketball on someone's finger."

"Windmill."

"House fan."

"Hamster wheel; Ferris wheel."

"Doorknob."

Describe an object that has both translational kinetic energy and rotational kinetic energy.
"Bicycle rolling down a hill."

A unicycle."

"A tire rolling down and in contact with a road."

From starting at the top of the ramp to the bottom of the ramp, indicate the changes in each of the energy forms of the tire.
(Only correct responses shown.)
Gravitational potential energy: decreases [84%]
Translational kinetic energy: increases [86%]
Rotational kinetic energy: increases [79%]

From starting at the top of the ramp to the bottom of the ramp, the energy form that experienced the greatest amount of change (increase or decrease) was the tire's:
 gravitational potential energy. ************** [14] translational kinetic energy. **** [4] rotational kinetic energy. ***** [5] (There is a tie.) ********** [10] (Unsure/lost/guessing/help!) **** [4]

For the subsequent part of this stunt, from just as it leaves the second ramp to reaching the top of its trajectory, indicate the changes in each of the energy forms of the tire.
(Only correct responses shown.)
Gravitational potential energy: increases [78%]
Translational kinetic energy: decreases [68%]
Rotational kinetic energy: no change [43%]

For the subsequent part of this stunt, from just as it leaves the second ramp to reaching the top of its trajectory, the energy form that experienced the greatest amount of change (increase or decrease) was the tire's:
 gravitational potential energy. ************** [14] translational kinetic energy. ****** [6] rotational kinetic energy. **** [4] (There is a tie.) ****** [6] (Unsure/lost/guessing/help!) ******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I found the tire rolling down the mega-ramp oddly satisfying."

"Go over how to find values for KEtr and KErot)."

"Could you use these equations to predict how a certain tire made at different masses would be ideally and most efficient at rolling?" (Yes, we can.)

"If energy can be converted from one form to the other does that mean all energy types should have the same SI unit?" (Yes!)

"I'm going to need a lot of review on this, I'm pretty confused..."

"Need lots of help with this in class!"

"I am desperate! I can't catch up the pace--I understand the content but it takes me forever to do all the exercises, read the book, blog and try to understand the lab beforehand...and I never procrastinate! :( Sad!"

"I need to work on my time management."