## 20171004

### Online reading assignment: energy conservation

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 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.
"Conservative forces do work that can store and later retrieve energy without loss, and nonconservative forces do work that irreversibly loses or gains energy."

"Gravitational potential energy and elastic potential energy and translational kinetic energy are all related. Together they make up the right side of the transfer/balance equation."

"The idea of stored energy somehow makes sense to me. Taking a couple of science classes where we learned about the stored energy in ATP and how it is spring loaded in a way."

"Kinetic energy is energy in motion. Potential energy is stored energy. Potential energy is most commonly determined by the change in height. Since gravity and mass stay constant, height is the most important determinant in change."

"I understand that there are conservative forces, such as gravitational weight and elasticity, that conserve energy by storing it in the system and then applying work to release the stored energy. Conversely, frictional and dragging forces are non-conservative, and energy is lost."

"Conservative forces are forces that store energy for later use. An example of this is the weight force of Earth on an object. Potential energy is that energy that changes position to be stored for later."

"Elastic potential energy is energy stored as a result of applying a force to deform an elastic object. The energy is stored until the force is removed and the object springs back to its original shape, doing work in the process."

"The complete form of the total energy conservation equation shows the full transfers between kinetic energy, along with potential elastic and gravitational energy. They all individually can be increasing or decreasing, but they all must result in the non-conservative work term."

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.
"The whole concept of non-conservative forces and conservative forces was confusing to me. It took me multiple readings of most of the passages in the online presentation just to get the gist of it. I understand it a little more now but it is still very confusing to me."

"I'm not exactly sure on this but when an object is in motion does it still posses potential energy or is there no potential energy and it has all been converted ot kinetic energy? can it posses potential elastic energy but still have no potential gravitational energy?"

"I'm struggling with the use of the equations for the energy types."

"I do not completely understand conservative force and its two types. Also I did not understand the gravitational potential energy equation."

"I don't understand exactly how to apply mechanical energy and know when it is conserved or when it isn't."

"Most everything in this chapter, starting from conservative and non-conservative energies, to elastic potential energy and the transfer/balance equation."

"Nothing was found too confusing."

"All I really understood this from reading assignment was how the work-energy theorem is used, and I;m still not 100% okay with how to apply it."

"Nothing was found too confusing."

"I feel like I understood this portion pretty well in that I feel confident with my answers to the reading assignment."

For the woman moving upwards after being catapulted, her translational kinetic energy __________ while her gravitational potential energy __________.
 decreases; increases. ********************************* [33] increases; decreases. ***** [5] (Unsure/lost/guessing/help!) ** [2]

For the ball bearing being launched by the slingshot, its translational kinetic energy __________ while the elastic potential energy of the slingshot bands __________.
 decreases; increases. *** [3] increases; decreases. ********************************** [34] (Unsure/lost/guessing/help!) *** [3]

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. [78%]
Gravitational potential energy: decreases. [70%]
Elastic potential energy (of the bungee cords): increases. [55%]

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. ******* [12] her gravitational potential energy. **************** [16] the elastic potential energy of the bungee cords. ******* [7] (Unsure/lost/guessing/help!) ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If I could ask you to go over one thing in the next lecture, it would be bungee-swinging woman. Thanks!"

"The last example got me, not as comfortable with horizontal motion being mixed in with vertical motion and determining which force is greatest."

"Go over conservative forces?"

"I would like some extra discussion explaining the different forces and energies. It just seems like maybe a little much at the moment."

"Could you explain a little bit more on elastic potential energy?"

"Very wordy reading assignment, read it over a few times and still can't understand it completely."

"Those equations hurt my eyes."

"Do we have to remember all of these equations?" (No, all the energy equations will be given to you, in the same format as the last page of the worksheet packet.)

"Would any object that has the potential to fall have potential energy? Does this work for objects that never actually do fall?" (Yes; but if they never fall their gravitational potential energy will just remain constant--it's like having a gift card ("potential money?"), but never getting around to use it.)

"I’m really sorry I haven’t been able to get to some assignments! I’m trying to balance everything better in my life, I hope you understand." (Just letting me know what kind of crazy is going on with your outside-of-physics life is still informative to me, and you still get completion credit for stuff like this.)

"I'm enjoying the class!"