## 20161024

### Online reading assignment: rotational dynamics

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 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 velocity is radians per second. Rotational kinetic energy is fixed about an axis and must have a moment of inertia."

"I appreciate how translational and kinetic energies are related. I have a fascination with kinetic energy and how it corresponds with other bodies."

"A rolling object's translational speed is determined by multiplying the radius of the object by the angular speed of the object. v = r·ω."

"That if two rolling objects have the same velocity, the one with a smaller radius will have a bigger angular speed, and the one with the larger radius will have a slower angular speed."

"How a rolling object with a smaller radius has a faster ω angular speed in order to keep up with an object with a larger r radius which will have a slower angular speed as it roles."

"That translation and rotational kinetic energy is related via velocity, and that rotational kinetic energy can also be figured into energy conservation equations."

"The inputs to the equations and what each equation represents, however I just need to see some examples and work done to fully grasp the material."

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 have never been great with calculating rotational kinetic energies. There's somethingoing about energy and inertia that I have difficulty with discerning when talking about something like a wheel."

"I don't understand what rotational energy is."

"I'm a little confused on the concept of inertia. What is inertia? How does it relate to angular speed?"

"I am having trouble understanding at which exact moment energy changes in a system."

"The difference between rotational energy and translational energy, how can they be independent of one another?"

"I am slightly confused on how an object can have both translational and rotational kinetic energy. The total energy conservation equation is a little confusing as well. I think seeing a problem worked out in class will be very helpful."

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

"kg·m2?"

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

"A beach ball rotating in place in a pool of water."

"A Ferris wheel"

"A windmill because it is not moving, just rotating."

"A hamster wheel that is stuck in place but cannot roll forwards or backwards or any other direction."

"Blades in a blender don't have translational energy because the blades only rotate and they aren't moving other than spinning around the axis."

"A clothes dryer."

"A wind turbine."

"Anything that will stay in place but rotate on an axis."

Describe an object that has both translational kinetic energy and rotational kinetic energy.
"Rolling a ball down an incline."

"A bicycle going down a hill."

"Earth"

"A rider on a Solowheel."

"A car is driving down a road, while the tire goes around it has a translational kinetic energy for how fast it is going, but it also has a rotational kinetic energy to show how fast the circular rotation is going."

"A launched ball spinning in the air."

"An object moving in a single direction as well as rotating around an axis."

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 [83%]
Translational kinetic energy: increases [74%]
Rotational kinetic energy: increases [78%]

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. ****** [6] rotational kinetic energy. ********* [9] (There is a tie.) ****** [6] (Unsure/lost/guessing/help!) ******** [8]

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 [74%]
Translational kinetic energy: decreases [50%]
Rotational kinetic energy: no change [39%]

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. *************** [15] translational kinetic energy. ********* [9] rotational kinetic energy. *** [3] (There is a tie.) ********** [10] (Unsure/lost/guessing/help!) ********* [9]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I think the gravitational potential energy should increase/decrease the most in both tire situations because KEtrans and KErot are both changing in the same 'direction' (increase/decrease) so gravitational PEgrav needs to change as much as those two added together meaning it should be the biggest. Is that wrong thinking?" (Yes, if the tire both rolled downhill, or back up a hill. But since the tire as it moves upwards is in mid-air, not rolling up a hill, the KErot doesn't change (its rotational speed stays constant.)

"Does a tire having 'rolling with slipping' need to be on a frictionless surface or a wet surface?" (Yes, or if the tire is "burning rubber" at the very start of a drag race.)

"Will we be practicing the tire problems in class?"

"Not sure how to apply the conservation of energy equations here."

"I would like some conceptual basics gone over in class and not just example problems."

"Examples would be helpful for relationship between rotational and translational kinetic energy."

"How can we tell which type of energy has the biggest change?"

"I need help on rotational kinetic energy."

"Will we learn about the physics of waves? Not sound waves but waves in the ocean or lake." (Primarily one-dimensional waves along strings/wires/ropes/cables, but those concepts can be extended to any kind of waves that travel through water or air.)