## 20160928

### Online reading assignment: energy conservation

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 energy conservation.

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.
"An object in motion has translational kinetic energy. The value of the gravitational potential energy will get larger or smaller depending on the height. Kinetic energy of motion can be converted into potential energy of position."

"Translational kinetic energy is in everything that has motion. Gravitational potential energy is a function of height above the surface. Elastic potential energy is energy that is a result of say, a spring or a rubber band."

"The energy of conservative forces can be retrieved without loss. Also non-conservative forces involve transfer or loss of energy when work is done."

"Gravitational potential energy depends on from where an object is dropped. The higher up an object is, the greater gravitational potential energy it has."

"As long as I know the variables in equation it should be easy."

"I think that I have really grasped the idea of conservative forces and how they first store energy from an object only to give it back. This causes gravitational and elastic forces to work in essentially the same way, and therefore have similar equations."

"Potential gravitatinal energy changes based on height and the elastic potential energy changes by the distance the spring/band stretches/contracts. I understand that these energies can be irreversibly lost to work done by friction and drag."

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.
"These sections related many equations to each other, further review will help to understand them easier."

"I am really struggling with the major concepts in these sections. Especially gravitational potential energy and kinetic energy."

"All the formulas seems very complex, all the subscripts. and the way the book is set up you only see them once. A little fuzzy on the non-conservative forces."

"I am unsure of how to figure out how gravitational potential energy works."

"Nothing yet, but I will be confused when I read it tomorrow."

"I am confused over the transfer/balance equation. Specifically, I don't understand how the kinetic, potential elastic, and potential gravitational energies have a relationship to the non-conservative work. Also why is it work zero when friction and drag are negligible?"

"I have trouble sometimes identifying where a force is. Also sometimes knowing which of Newton's laws is used."

"I am a little confused about when these conservative forces act together with non-conservative forces, because the equation is really long. I guess it's just the transfer/balance equation that confuses me."

"The practice problems at the bottom of the blog made no sense to me at all. I couldn't understand how to interrelate all of the different formulas."

"I found the different formulas a little confusing. If we could do some examples in class to clear it up that would be sweet."

"I just think applying the equations and knowing when to use them is the hardest part. Its hard to read something for the first time and see the scary equation and apply it all in my head. But that's why we go over it in class!"

"I understood the differences between conservative and non-conservative forces, but still am really confused how to get the formulas based off of them and to derive one variable out of an equation."

"There is a lot that I don't understand so the more explanation in class the better."

"I'm having trouble understanding what stored energy is. I can't really fully understand the concept of it."

"I didn't find anything confusing."

For the woman moving upwards after being catapulted, her translational kinetic energy __________ while her gravitational potential energy __________.
 decreases; increases. ******************************** [32] increases; decreases. ******* [7] (Unsure/lost/guessing/help!) ******* [7]

For the ball bearing being launched by the slingshot, its translational kinetic energy __________ while the elastic potential energy of the slingshot bands __________.
 decreases; increases. **** [4] increases; decreases. *********************************** [35] (Unsure/lost/guessing/help!) ******* [7]

For the woman falling off the building starting from the edge of the roof to just before reaching her lowest point of descent, indicate the changes in each her of energy forms. (Only correct responses shown.)
Translational kinetic energy: increases. [54%]
Gravitational potential energy: decreases. [59%]
Elastic potential energy (of the bungee cords): increases. [63%]

For the woman falling off the building starting from the edge of the roof to just before reaching her lowest point of descent, the energy form that experienced the greatest amount of change (increase or decrease) was:
 her translational kinetic energy. ********** [10] her gravitational potential energy. *************** [15] the elastic potential energy of the bungee cords. ************ [12] (Unsure/lost/guessing/help!) ********* [9]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"How is translational kinetic energy and gravitational potential energy different than heat or light?" (Here we're looking at mechanical energy systems of objects on the macroscopic scale; heat and light are forms of energy on the atomic scale.)

"I do not understand how translational kinetic energy increases or decreases. If we look at it over an interval, does it count at the beginning or the end?" (There is both an initial KEtrans value and a final KEtrans value. But more interesting is the amount of ∆KEtrans change, and how it increases or decreases for a process.)

"How can the total mechanical energy of a moving object remain constant, provided that the net work done by external nonconservative forces is zero? How can  the net work done by external nonconservative forces equal zero?" ((a) If the energy forms only trade with each other, and not with the outside world; and (b) if any losses in energy by drag and friction work are offset by gains in energy put in by applied forces pushing/pulling the object forward.)

"More discussion on what makes a force conservative vs. non-conservative." (Good. Then we get to talk about sliding that cat across the floor. I like talking about cats.)

"Can we make one of those bungee swings from the top of the library?" (Who wants to be the first to test it out?)

"If we have more than one energy type in an equation, what do we do?" (I'll show you what we do in cases like that. Basically balance out (or try to balance out) the transfers between the different energy types. Or numerically plug-and-chug.)

"What would be the best way to memorize and understand these complicated-looking formulas?" (You're given all the equations for work and energy, in class today we'll take a closer look at using these equations both conceptually and in problem-solving.)

"I always understand how everything works when you go over it in class, and then I go back and look at how I solved problems in class and it looks like a different language. This is why the midterm is scary. :0" (That's normal when you're just getting started on the new stuff. We'll go back and review the old stuff next week that will be on the midterm, hopefully by then it will seem more familiar and less scary.)