## 20141020

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 collisions.

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.
"I understand the different types of collisions. I also understand how to use the conservation law equations."

"Total momentum is a sum total of momenta of objects in a system. Internal interactions do not change total momentum, external interactions might change it. Reading also touches on what a collision is."

"A perfectly inelastic collision occurs when two objects stick together. Inelastic occurs when one object pushes the other away and there's visible damage. Finally, an elastic collision occurs when one object completely pushes another and there is no visible damage."

"I assumed that two objects stuck together wouldn't lose lots of energy, but after reading more about it, they can lose lots of kinetic energy."

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 relative velocity kinetic energy conservation equation just is not clicking for me. A brief explanation of why that equation is what it is would help greatly."

"You lost me at the concept of momentum."

Explain the difference between a (partially) inelastic collision and a perfectly inelastic collision.
"If two cars hit each other and do not stick to each other , but are crushed or bent, then the collision is partially inelastic. If two cars collide and are stuck together, the collision is perfectly inelastic."

"In an inelastic collision both objects bounce off each other and there is visible damage, in a perfectly inelastic collision, however, both objects are stuck together in addition to visible damage."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.
"We're limiting it to the initial state just before the collision and the final state just after the collision. The external forces have negligible impulses in the short time span."

"The system needs to be isolated from all external factors, because if it isn't then momentum is not conserved. To put it into context, external forces will always be a factor, but the forces involved within a collision are so large that any external forces can be safely ignored, for they do not alter the final result in a significant way."

For the Nissan Altima and Nissan Rogue crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Perfectly inelastic (kinetic energy not conserved). ** [2] (Partially) inelastic (kinetic energy not conserved). **************************************** [40] Elastic (kinetic energy conserved). ***** [5] (Unsure/lost/guessing/help!) ******** [8]

For the Ford Explorer and Ford Taurus crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Perfectly inelastic (kinetic energy not conserved). ******** [8] (Partially) inelastic (kinetic energy not conserved). ****** [6] Elastic (kinetic energy conserved). ********************************* [33] (Unsure/lost/guessing/help!) ******** [8]

For the train and minivan crash, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Perfectly inelastic (kinetic energy not conserved). ******************************* [31] (Partially) inelastic (kinetic energy not conserved). ******* [7] Elastic (kinetic energy conserved). ***** [5] (Unsure/lost/guessing/help!) ************ [12]

For the bullet burrowing through and back out of the baseball, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Perfectly inelastic (kinetic energy not conserved). *** [3] (Partially) inelastic (kinetic energy not conserved). **************************** [28] Elastic (kinetic energy conserved). ************ [12] (Unsure/lost/guessing/help!) ************ [12]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Perfectly inelastic (kinetic energy not conserved). *** [3] (Partially) inelastic (kinetic energy not conserved). ****************** [18] Elastic (kinetic energy conserved). ******************** [20] (Unsure/lost/guessing/help!) ************** [14]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I am having some trouble understanding elastic, partially inelastic, and perfectly inelastic collisions, and momentum conservation and kinetic energy conservation equations. Would you be able to go over some examples in class please? Could you also go over kinetic energy being conserved or not being conserved?" (Yes. And if time allows, you'll get to solve an example as well.)

"How do you tell the difference between perfectly inelastic and partially inelastic collisions? Is it just the sticking (or not)?" (Yes, for a perfectly inelastic collision, the objects are stuck-together after the collision. It's actually quite common in automobile accidents.)

"What would it be if two objects collided, stuck together, and did not have deformation?" (If there is not visible deformation, then translational kinetic energy was dissipated into other forms, such as sound, heat, etc.)

"Can you go over the last question on the midterm?" (It's discussed on the blog, along with samples of student solutions.)

"Do you celebrate Halloween?" (My people celebrate something called "Madonnaween.")