20191021

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 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.
"This section is about rolling objects in terms of rolling dynamics, rotational inertia, and angular speed. With these ideas the reading linked them to previous ideas of potential and kinetic energy."

"I somewhat understand how to recognize changes (increases/decreases) in rotational kinetic energy, and how rotational speed ω is related to translational speed v for objects that roll without slipping."

"The concept of balancing the angular speed equation v =r·ω. Larger objects have larger radii and therefore must have a smaller ω to match the velocity of a smaller object."

"As a circular object rolls down a hill without slipping, its gravitational potential energy decreases at the same rate that its (translational kinetic energy + rotational kinetic energy) INCREASES. I also understand that when a circular object that doesn't slip launches off of a ramp, it will maintain the SAME rotational kinetic energy in the air that it had at the moment it left the ramp."

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.
"Moment of inertia to be confusing."

"Rotational inertia and inertia in general. I'm also confused on how we relate inertia and energy in the equations given."

"How to find the variables in the equations, they all look like random letters and I don't know what they stand for."

"I think I just need some review to further understand the difference between equations and how to memorize what equation to actually use in a given situation. Otherwise, I feel confident on this subject."

"The concept of rolling motion is still confusing. I'm unsure of what tangential speed really is."

"I'm trying to wrap my head around the rolling motion."

"I would like a little more explanation on all the terms in the energy transfer-balance equation."

"Upon completing the presentation preview, I was very confused about the rotational kinetic energy at first but soon figured it out once given another glance. The rotational kinetic energy depends on rotational inertia and the angular speed of the object."

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

"Like other forms of energy, the SI unit for rotational kinetic energy is the joule (J)."

"kg·m2/s2."

Describe an object that only has rotational kinetic energy, and no translational kinetic energy.
"A ceiling fan is an object that has only rotational kinetic energy and no translational kinetic energy."

"A windmill/wind turbine."

"The gear in a watch or machine that only spins and stays stationary."

"A hamster wheel has rotational kinetic energy but no translational kinetic energy."

"It would be an object that continues to spin, but remains in one place, like a Ferris wheel."

"An ice skater spinning in one place would only have rotational kinetic energy but not translational. This is because they are only rotating about an axis, but are not moving through space from one point to another."

"An object can rotate really really fast but its rotational kinetic energy can still be zero as long as it does not have any contact with a solid surface. Example: a car at an auto shop is resting on a lift like in the movie Feris Bueller's Day Off, and the rear wheels are rotating really fast, but since there no contact with the ground the car is not moving anywhere and transitional kinetic energy is zero."

"Any wheel that spins around a fixed axis."

"I'm not 100% on understanding this but I think a compact disc would have rotational kinetic energy but not translational because it isn't moving."

"I'm not sure on this one."

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

"The wheels on a skateboard which are both rotating and moving forward."

"A unicycle."

"A car driving on the walls of a circle."

"The teacup ride at Disneyland is an example of an object with both rotational and translational kinetic energies because it is rotating about an axis as well as moving from one point in space to another."

"I need further instruction on this."

"I am not sure."

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 [76%]
Translational kinetic energy: increases [78%]
Rotational kinetic energy: increases [80%]

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. ****************** [18] translational kinetic energy. ***** [5] rotational kinetic energy. ********* [9] (There is a tie.) ******* [7] (Unsure/lost/guessing/help!) ******* [7]

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 [76%]
Translational kinetic energy: decreases [57%]
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. ****************** [18] translational kinetic energy. ********* [9] rotational kinetic energy. *** [3] (There is a tie.) ******** [8] (Unsure/lost/guessing/help!) ******** [8]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we go over specific examples for each type of energy showing when each energy increases and decrease?"

"I am EXTREMELY confused how to tell the difference between the gravitational, rotational, and translational energies. May we have a quick simple break down summary in class?"

"I'm liking the colder weather."

"Q: How does a German physicist drink beer? A: With ein Stein."