## 20171023

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 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.
A completely elastic collision is where the translational kinetic energy has been completely conserved. A completely inelastic collision has almost all of the kinetic energy transformed into permanent deformation."

"Inelastic collisions can either be partial or completely inelastic. The difference lies between whether or not they both bounce back away from each other. All inelastic collisions cause deformation and therefore lose energy and momentum within the system."

"Elastic collisions cause no physical damage to the objects and kinetic energy is conserved. Inelastic collisions do not conserve kinetic energy and damages at least one of the objects involved."

"If there is no rebound, then its inelastic, if it's partially off, then its partially inelastic and if there was no damage than its elastic."

"The concept of internal and external forces. How internal forces are the forces done by the system that are released upon each other. While external forces are the forces exerted on the objects/system by what is around the system itself."

"The collision type flow chart and the kinetic energy conservation flow chart makes a lot of sense. I would say I understand the difference between each of the collisions and whether kinetic energy is conserved."

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 flowchart in the online blog part of the reading was a little confusing at first, but I was able to essentially memorize it after staring at it for long enough."

"The different types of collisions."

"How would you go about figuring out if a collision was going to be elastic or inelastic? Could you also figure out the force that is being exerted by the objects?"

"The flowchart in the online blog part of the reading was a little confusing at first, but I was able to essentially memorize it after staring at it for long enough."

"I'm a little confused about the bullet passing through the baseball."

"The conservation of kinetic energy is a little confusing for this. I can definitely use some help with the formulas and equations."

"What is considered and applied in the situations of external and external forces I found somewhat confusing. As an internal force isn't necessarily something you can track in a sense."

"I would appreciate a brief review of momentum conservation. I think the equation is slightly confusing."

"I didn't find one thing in particular confusing, but I would enjoy some more explanation and review of the information in the presentation. Some example problems to show application would be helpful too."

"I didn't seem to have any problems with this section as the concepts are pretty straightforward. I think I will only really need practice in how to apply it to an example."

"Everything was clear!"

"I like this chapter."

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.
"a partially inelastic collision and a completely inelastic collision differ because a partially inelastic collision does not have the two objects stuck together, but still has permanent deformation."

"A partially inelastic collision is when the objects rebound. While complete inelastic is they stick together."

"A completely elastic collision is one in which the two objects that crash move together as one. A partially inelastic collision is one in which the two objects cause damage, but are separated form each other."

"Partially inelastic collision is when two objects collide they release from one another, but are deformed in some way. While complete inelastic collision is when two object collide together and stick together."

"Whether the objects are stuck together or apart with lasting damage."

"I have no idea."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.
"We are limiting our discussion to the initial state before the collision and the final state directly after the collision since it all happens in fractions of a second."

"Since its a brief collision there's such a small amount of time for any of those forces to become impactful."

"Their magnitudes are very small compared to the other forces at play in the collisions."

"I don't know the answer to this."

"Having trouble with this concept."

"I am not sure."

For the Nissan Altima and Nissan Rogue crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. [0] (Partially) inelastic. *************************************** [39] Elastic. * [1] (Unsure/lost/guessing/help!) [0]

For the Ford Explorer and Ford Taurus crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. * [1] (Partially) inelastic. ****** [6] Elastic. ********************************* [33] (Unsure/lost/guessing/help!) [0]

For the train and minivan crash, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. ************************************ [36] (Partially) inelastic. ** [2] Elastic. ** [2] (Unsure/lost/guessing/help!) [0]

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.)
 Completely inelastic. ****** [6] (Partially) inelastic. ************************ [24] Elastic. ******* [7] (Unsure/lost/guessing/help!) *** [3]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. [3] (Partially) inelastic. ******************** [20] Elastic. **************** [16] (Unsure/lost/guessing/help!) * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"How many equations are there for these types of problems?" (If momentum is conserved, just one equation.)

"Applications and definitions in normal words would help."

"The flowcharts really help me try to find a baseline of information for each new concept!"

"I think I get this. :) Also loved these short videos!"