## 20141027

### Online reading assignment: rotational dynamics

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 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.
"Rotational kinetic energy is being added to the energy transfer equation."

"How rolling objects combine rotational kinetic energy with translational kinetic energy as they move."

"I takes the place of mass and ω takes place of speed in rotational kinetic energy. This creates a formula that is similar to translational kinetic energy but only with the m and v swapped out."

"A rolling object's translational speed v and angular speed ω are constrained via the "rolling without slipping" condition v = r·ω, so an object with a radius r that has a slow or fast translational speed must also be rolling with a corresponding slow or fast angular speed."

"That translational kinetic energy and rotational kinetic energy are different. I got that much."

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 was not particularly confused by anything in the reading."

"Rotational kinetic energy and determining if it is increasing or decreasing. And how to calculate rotational inertia."

"'For an object with both translational motion and rotational motion, it will have both translational kinetic energy and rotational kinetic energy'--I believe this concept needs a little more explanation before it sinks in."

"What is the lower-case Greek letter ω stand for? Does it stand for the rotational speed or something else?"

"When the different types of energy are increasing or decreasing. Can we go over examples of when each energy increases and decrease?"

"Rotational inertia--there are many different formulas for the different objects and I don't fully understand how to apply them in problems."

What is the SI (Système International) unit for rotational kinetic energy?
"J."

"kg·m2."

"I."

"Krot."

"N/v."

Describe an object that only has rotational kinetic energy, and no translational kinetic energy.
"A hamster wheel."

"A wind turbine or a fan has no translational kinetic energy, as it is fixed to a central axis that is stationary."

"A Ferris Wheel."

"Toilet paper dispenser."

"Any object that is spinning, but not moving."

"A CD spinning in a CD player."

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

"The boulder chasing Indiana Jones."

"A wheel on a car."

"A unicycle, which is spinning and moving."

"The tire rolling down the ski jump?"

"A bicycle."

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 [78%]
Translational kinetic energy: increases [62%]
Rotational kinetic energy: increases [91%]

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. ***************** [17] translational kinetic energy. ** [2] rotational kinetic energy. ************** [14] (There is a tie.) **************** [16] (Unsure/lost/guessing/help!) ****** [6]

For the subsequent part of this stunt, from starting at the bottom of 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 [76%]
Translational kinetic energy: decreases [40%]
Rotational kinetic energy: no change [33%]

For the subsequent part of this stunt, from starting at the bottom of 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. ***************************** [29] translational kinetic energy. **** [4] rotational kinetic energy. **** [4] (There is a tie.) ************ [12] (Unsure/lost/guessing/help!) ****** [6]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why does my spell check say 'translational' is not a word?"

"How is the rotational kinetic energy affected when the tire is in the air? Does it still meet that 'no slipping' requirement from before?" (No, it does not--which means that the angular speed of the tire will ideally be constant, even though the translational speed changes as it would for projectile motion.)

"Why do you not include any mathematical examples of the ideas in the blogs?" (That's what the textbook is for. The blog presentations are to emphasize the important concepts over the extraneous topics in the textbook that will not appear on quizzes and exams.)

"Can we go over specific examples for each type of energy showing when each energy increases and decrease? Examples using the rotational and translational equations would be helpful." (Yes, as time allows. Don't take too long with the "scratcher" group quiz.)

"I'm coming to class!!!!" (Uh, okay.)