20131020

Online reading assignment: collisions

Physics 205A, fall semester 2013
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.
"Different kinds of collisions. I like watching things being destroyed."

"I found it interesting that two objects stuck together after collision lose lots of kinetic energy. I thought it would be opposite."

"The older car versus new car collision makes me never want to drive an older car."

"The difference between perfectly inelastic and inelastic collisions."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I don't understand how to tell if momentum was conserved."

"I found the momentum and kinetic energy being conserved or not a little confusing. It was confusing because I was having trouble remembering when something was being conserved or not."

"I am a little bit confused on why we are able to neglect external forces during a collision. The textbook explains that interactions between two objects can be considered internal forces, and thus the change in momentum is within the system and not considered to be exerted by an external force. I would benefit from some clarification of internal and external forces."

Explain the difference between an inelastic collision and a perfectly inelastic collision.
"In an inelastic collision, the two objects collide and bounce off each other and some kinetic energy is transferred into damage. In a perfectly inelastic collision the objects collide and are stuck together and lots of kinetic energy is lost."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.
"Over the very short duration of a collision the net force effect of drag, friction, etc. are insignificant."

Classify the bullet passing through the baseball collision (*.gif). (Neglect drag/friction/external forces during this "brief" collision.)
Elastic.  ************* [13]
Inelastic.  ************************** [26]
Perfectly inelastic.  ** [2]
(Unsure/guessing/lost/help!)  ****** [6]

What is conserved for the bullet passing through the baseball collision (*.gif)? (Neglect drag/friction/external forces during this "brief" collision.)
(Only correct responses shown.)
Momentum: conserved [81%]
Kinetic energy: not conserved [39%]

What is conserved for the bullet exiting the gun (*.gif)? (Neglect drag/friction/external forces during this "brief" collision.)
(Only correct responses shown.)
Momentum: conserved [83%]
Kinetic energy: not conserved [41%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The GIF animations for this section really helped!" (I love GIF animations.)

"I really would benefit from the correct answers to these after completion. Also, can you provide answers to the flashcard questions that we don't go over in class?" (In the time allotted in class, we'll prioritize going over the most problematic questions. If you need more discussion, ask during problem-solving session in class, come to office hours, or e-mail me.)

"I like the fact that you're incorporating your blog more into the reading assignments. I make a more conscious effort to understand your presentations." (I'm trying to get better at this class-flipping method.)

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