## 20121021

Physics 205A, fall semester 2012
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 collisions.

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"Collisions--I am a person that can't look away from a car crash, and I love Mythbusters, and watching things crash together, so that interests me."

"Only external interactions can change the total momentum in a system."

"How momentum is unchanged with a closed system only transferred."

"If you know muzzle velocity and mass of a bullet, and the mass of the gun you can calculate the recoil velocity. I shot a pretty freakin' large rifle in Africa, and it definitely had some recoil!"
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Having a hard time applying the momentum, impulse, and impulse-momentum theorem to the problems. I can't seem to figure out how to set them up."

"An isolated, or closed, system. This doesn't make too much sense to me because I don't see how this could actually happen--there are always going to be external forces acting on an object. Unless the object is in a zero-gravity, airless vacuum."

"Why linear momentum is always conserved for an isolated system."

"Keeping all the collision terms straight in my head. I kept mixing up what was elastic, perfectly inelastic, partially inelastic. The flow charts in the presentation helped me sort them out, but I still get the terms confused."

"So far, I am not sure how to do one of these kinds of problems because I have not known how to do one of these problems."

"I had assumed that when two objects collide, there would be some kinetic energy lost somehow. This isn't actually the case."
Briefly explain the conditions/assumptions under which momentum will be conserved for a collision.
"Frictionless surface, no air resistance."

"When external forces are zero. When there are no external forces involved."

"Collision must be brief where initial and final states are just before and after the collision (such that external impulse approaches 0)."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Please go over momentum conservation." (Definitely.)

"Did you say earlier in the semester that if we had an 'A' going into the final exam that we didn't have to take it or was that from another class? I hope it was this one." (This is the class!)

"This seems way easier than the stuff we were learning before." (It does?)