## 20191014

Physics 205A, fall semester 2019
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.
""Total momentum of an isolated system is the same before and after a collision. An isolated system is which the vector sum of the average external impulses acting on the system is zero."

"There are three ways to identify collisions: elastic, inelastic, and completely inelastic. They are classified according to whether the total kinetic energy changes due to the collision."

"There are three types of collisions: elastic, partially elastic, and completely inelastic. Kinetic energy is only conserved in elastic collisions because there is no permanent deformation of the objects colliding."

"When objects collide and rebound off each other without causing damage the energy is conserved. However, when two objects collide and are crushed or bent, kinetic energy is lost as heat or from deformation."

"An elastic collision is when there is no damage. Partial inelastic means there is crushing or damage but the objects are no longer touching. Finally complete elastic is when there is damage and the objects are still touching."

"In an elastic collision, the two objects rebound off each other, and ideally there is no permanent damage deformation afterwards. Even though there is no cosmetic damage in this collision there was minor structural damage, but this is close to a ideal elastic collision."

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 am confused on where the kinetic energy is exactly transferred to, depending on if it's an elastic or inelastic collision."

"How kinetic energy is related to the different types of collisions. I also don't understand momentum conservation."

"I found the principle of conservation of linear momentum to be confusing for me. The textbook it states 'the total linear momentum of an isolated system remains constant. An isolated system is one for which the vector sum of the average external forces acting on the system is zero.' Something about this definition just did not fully click for me and I'm hoping lecture will help me to gain a better understanding."

"I don't understand anything about conservation of momentum. I don't understand how the internal and external forces affect the collision, or when the net force is zero or not. I also don't understand how to tell if the kinetic energy is the same or not before/ after a collision."

"The mathematical analysis of collisions are difficult to interpret correctly."

"I found using the equations confusing especially when deciding when a velocity is negative or positive."

"I'm confused on how to use the momentum and kinetic energy conservation equations."

"Is momentum the same as kinetic energy?"

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.
"In a partially inelastic collision, the two objects collide and rebound off each other but there is permanent damage done afterwards. In a completely inelastic collision, the two objects will collide and stick together."

"With both inelastic collision types, the kinetic energy of the system after the collision is not the same as the kinetic energy of the system before a collision and the objects still separate, but with completely inelastic the objects will stick together."

"Partially inelastic results in damage to the objects but they do not become stuck. Completely inelastic is when the objects become stuck and are physically altered. More damage means more kinetic energy loss."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.
"The contact forces during collisions are so large compared to the external forces such as friction and air resistance that we neglect them."

"The drag, friction, and other external forces do not matter during sufficiently 'brief' collisions because those impulses would be almost negligible in at such a short time period."

"They are negligible due to the relatively brief time that collisions take place, so if we look at the left (external impulse) side of the momentum conservation equation we can set it equal to zero, so the objects are exchanging momentum."

"I don't fully understand to be honest."

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. ********************************** [34] Elastic. ** [2] (Unsure/lost/guessing/help!) ** [2]

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. ** [2] (Partially) inelastic. ****** [6] Elastic. **************************** [28] (Unsure/lost/guessing/help!) ** [2]

For the train and minivan crash, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. **************************** [28] (Partially) inelastic. *** [3] Elastic. ***** [5] (Unsure/lost/guessing/help!) ** [2]

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. ***** [5] (Partially) inelastic. *************************** [27] Elastic. *** [3] (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. ************ [12] Elastic. ***************** [17] (Unsure/lost/guessing/help!) ****** [6]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Which type of collision consists of one object being damaged but the other object was not altered in any way?" (Even if only one object was damaged, then that would make it either a partially inelastic collision (if they separated afterwards) or a completely inelastic collision (if they stuck together afterwards). An elastic collision is only if both objects are undamaged and separated afterwards.)

"Is there such a thing as a partially elastic collision?" (That would be covered under the partially inelastic collision category.)

"Can a collision actually be elastic even with drag air resistance?" (On the smallest scale, gas atoms and molecules bounce off each other elastically with no energy loss.)

"It was harder than I thought to determine which one of those were elastic or inelastic or completely inelastic."

"Will we be looking at car crashes and how it relates to the three types of collisions in lab?" (Yes, specifically the completely inelastic collision type.)

"I'm a little confused when deciding whether or not a collision is completely inelastic or partially inelastic. What happens if one object goes completely through the other?" (As in the bullet goes into the baseball and comes out the other side, there is permanent damage done, so some kinetic energy was lost, such that this makes it a (partially) inelastic collision.)

"I think this is the first thing I have somewhat understood all semester so far."