## 20141001

### Online reading assignment: energy conservation

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 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.
"Some forces can be classified as being conservative. These forces are able to store energy and use it later without loss of that stored energy."

"Gravitational potential energy increases as the object gets higher up and decreases as it gets lower to the ground. Elastic energy increases as an object is stretched."

"Potential energy is energy that is stored by moving to a different position; if you climb higher, gravitational potential energy will increase until you fall again. Pull an elastic band back, the energy will be stored until it can snap back."

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 found all the equations like translational kinetic energy, gravitational energy, and gravitational potential energy confusing. I would not know how to solve an equation regarding these equations."

"Kinetic energy comes from potential energy, so I'm still a little skeptical on how that energy equation works."

"I think it is confusing to keep straight all the different forces and energies there are and what they do. It would be nice to have a review of the individual forces and energies and what they act on."

For the woman moving upwards after being catapulted, her translational kinetic energy __________ while her gravitational potential energy __________.
 decreases; increases. ********************************** [34] increases; decreases. ***************** [17] (Unsure/lost/guessing/help!) *** [3]

For the ball bearing being launched by the slingshot, its translational kinetic energy __________ while the elastic potential energy of the slingshot bands __________.
 decreases; increases. ********* [9] increases; decreases. *************************************** [39] (Unsure/lost/guessing/help!) ****** [6]

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. [63%]
Gravitational potential energy: decreases. [67%]
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. ************ [12] her gravitational potential energy. ******************* [19] the elastic potential energy of the bungee cords. *************** [15] (Unsure/lost/guessing/help!) ******** [8]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"What happens if this section is blank when I don't have any real questions? Do I still get credit even if I just mark that I haven't read anything and am unsure about everything?" (You get credit for 'completing' this assignment, so if the earlier sections are left blank (or effectively blank), you should leave something substantive (even if it's just describing the extent of your confusion) in the question/comment box in order to receive full credit.)

"In the change in gravitational potential energy equation: do we always use the magnitude |g|?" (Yes. The positive or negative sign of ∆Ugrav comes only from the ∆y term.)

"Please go over some examples of translational kinetic energy, gravitational potential energy, and elastic potential energy in class so we could get a better understanding." (Yes, we can.)

"That homemade bungee jump looks insane! I would never try that, even if you paid me with a lifetime supply of beef jerky." (What about two lifetime supplies of beef jerky?)

"Where do you get these GIF animations?" (I extract them from YouTube using a Firefox download plug-in, and use a movie-to-GIF converter in order to deliver them to your eyeballs.)

"How hard is it to take care of your goatee? It is the most awesome goatee ever."