Online reading assignment: collisions

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."

Online reading assignment: collisions

Physics 205A, fall semester 2018
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.

"There are three different types of collisions: completely inelastic, partially inelastic, and elastic. And each of these collisions can either conserve or not conserve kinetic energy."

"An inelastic collision is where the kinetic energy is not conserved. A completely inelastic collision is where it sticks together afterward, while partially inelastic collisions deform but don't stick together afterwards."

"The differences between inelastic and elastic collisions and how to identify them. Inelastic collisions (partial or complete) do not conserve kinetic energy, while elastic collisions do."

"An elastic collision is where two objects rebound and there is no permanent damage. Also, only elastic collisions conserve kinetic energy since there is no damage."

"Momentum is conserved for any collision as long as it is brief, the time is so short it becomes zero. Kinetic energy is conserved during elastic conclusions."

"When collisions happen during a very short period of time the left side of the momentum conservation equation can be set to 0. Basically when you are looking at a collision that happens in a short period of time you are just examining the exchange in momentum between the two objects."

"The different types of collisions and the rules for colisions. The conservation laws and the flowchart for them made perfect sense to me."

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 presentation was hard to understand. Also, the equation for momentum conservation look intimidating."

"I was confused by the principle of conservation of linear momentum."

"The equation for the conservations kind of confusing. I would just like to see examples worked out in class."

"I had a little trouble understanding the difference between elastic and inelastic collision. Elastic collision is when two objects collide head on and no heat or other form of energy is produced during the collision. Inelastic collision have some thermal and sound energy produced so kinetic energy is not conserved?"

"Categorizing collisions and determining kinetic energy conservation (I did read kinetic energy is only conserved for elastic collisions because none is permanently lost in deformation). Applying conservation law(s) for collisions."

"What I found confusing was why partially inelastic has kinetic energy lost but I think I understand it now."

"When we read the question it is very important to understand what happens after colliding. Sometimes I get confused about this."

"I understood everything in the presentation/textbook...for now anyway."

"I actually understood what this reading assignment was about."

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.

"A partially inelastic collision is where two objects bounce off of each other. A completely inelastic collision is where objects do stick to each other. Permanent damage is present in both cases."

"A partially inelastic collision is when two objects rebound off each other but there is some permanent damage afterwards and a completely inelastic collision is when two objects collide but they stick together."

"A partially inelastic collision has a small rebound with damage and a completely elastic collision has no rebound and the two crashing objects become 'stuck' to each other."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.

"The time is so small that the friction is negligent."

"We can neglect their impulses because the brief collision occurs in an instant, so δt would be zero."

"External forces/impulses would be negligible in the relatively brief time that collisions occur."

"Because collisions happen in such a brief time, external forces do not factor."

"These 'brief' collisions happen in a fraction of a second so even though these forces can be quite large their impulses are negligible because of the amount of time they take place in. So momentum is conserved because the objects will only exchange momentum."

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.	* [1]
(Partially) inelastic.	********************************* [33]
Elastic.	** [2]
(Unsure/lost/guessing/help!)	* [1]

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.	***** [5]
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.	******************************* [31]
(Partially) inelastic.	*** [3]
Elastic.	** [2]
(Unsure/lost/guessing/help!)	* [1]

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.	*** [3]
(Partially) inelastic.	*********************** [23]
Elastic.	******* [7]
(Unsure/lost/guessing/help!)	**** [4]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	***** [5]
(Partially) inelastic.	************ [12]
Elastic.	*************** [15]
(Unsure/lost/guessing/help!)	***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"How can you tell if something gets damaged during a collision, such as when firing a gun? Should we just have real-world knowledge or should we know this from examples?" (You'll practice this in class; basically any process that removes or adds kinetic energy counts as "damage.")

What would be the collision if the object went through the other object, for example a bullet going through another object." (That counts as "damage," and since the bullet and object are not stuck together afterwards, then this would be a partially inelastic collision. (If the bullet was embedded in the object, such that they are "stuck together," then this would be a completely inelastic collision.)

"Kinetic energy is all lost when you have completely inelastic collision?" (Well, not all of it is lost, but enough of it is lost such that the objects stick together (rather than rebounding) after the collision, even if they both still have some motion together afterwards.)

Online reading assignment: collisions

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!"

"What would you recommend for better preparing for the next midterm?" (I was just about to ask the class about this, for the "Keep-Quit-Start" mid-semester resolution survey.)

"Where can we find the extra-credit surveys?" (Look under Tuesday's homework. Also don't forget that each group "scratcher" quiz in class is also extra-credit.)

Online reading assignment: collisions

Physics 205A, fall semester 2016
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.

"This section was on collisions. There are three types, completely inelastic, partially inelastic, and elastic. Completely inelastic collisions will stick together. For inelastic collisions the two objects rebound off each other, and elastic collisions are when they rebound and there is no permanent damage. Elastic collisions are the only type where kinetic energy is conserved."

"There are three types of collisions in this section. Inelastic, in which two objects stick together, partially inelastic, in which two objects rebound off each other while inflicting damage, and elastic, in which two objects rebound off each other without causing permanent deformation."

"When a collision occurs and the objects stick together all of the kinetic energy is lost, this is complete inelasticity. If a collision occurs and there is some rebound off the other object and damage is smaller, then it is partially inelastic, but kinetic energy is still lost. When two objects bounce off of each other with no damage kinetic energy is conserved."

"It made sense to me how kinetic energy can only be conserved for elastic collisions; no damage is done in this type of collision and the two objects just bounce off one another, as opposed to crashing into one another and deforming each other in the two types of inelastic collisions."

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 found this section fairly straightforward. Momentum conservation was a little confusing to me, as well as using the calculation for it."

"The principle of conservation of linear momentum. More specifically, the breakdown of it and how it can be applied."

"Although I understand the concept of the collision types, the flow chart in the presentation sort of confused me. Is that just supposed to be a reference for help?"

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.

"Partially inelastic collision the two objects rebound off each other and there is no permanent damage. With a completely inelastic collision, the objects stick together."

"In partially inelastic the cars are not stuck together like they are in completely inelastic. There is permanent damage in both these collisions."

"Completely inelastic collison is when most kinetic energy is lost, objects stick together and damage is done. Partially inelastic collison is when some kinetic energy is lost, damage is done but objects separate after."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.

"With car collisions, if the external friction forces are large, their impulse would be negligible due to the relatively brief time that collisions have."

"Their impulse would be negligible due to the relatively brief time that collisions take place in. We can set the change in time equal to zero, such that the objects are simply exchanging momentum."

"It's just the initial and final, nothing in-between?"

"I am not sure about this."

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.	*** [3]
(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.	* [1]
(Partially) inelastic.	**** [4]
Elastic.	******************************** [32]
(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.	***************************** [29]
(Partially) inelastic.	***** [5]
Elastic.	[1]
(Unsure/lost/guessing/help!)	**** [4]

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.	********************** [22]
Elastic.	[4]
(Unsure/lost/guessing/help!)	******* [7]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	**** [4]
(Partially) inelastic.	********** [10]
Elastic.	******************** [20]
(Unsure/lost/guessing/help!)	***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"For inelastic collisions, does the permanent damage have to be to both objects, or can there be permanent damage to just one of the objects? For the train-car example, since the train did not undergo any structural damage, is it is elastic for the train and inelastic for the car?" (Any permanent damage/deformation means that translational kinetic energy was lost, whether it was to one or both of the colliding objects, and so the collision would be classified as one of the two inelastic types.)

"Can we get some clarification what constitutes 'permanent damage?'" (Any evidence that shows that a collision took place. Be sure to ask your auto body specialist.)

For the elephant gun example, are we assuming there was no permanent deformation of the man's body (bruising, dislocated shoulder, etc.)? I don't know if translational kinetic energy was conserved to make this elastic or partially inelastic." (Consider the amount of translational kinetic energy before the gun was fired (zero, since everything was stationary), compared to afterwards, where the man, rifle, and bullet were all moving. Translational kinetic energy was definitely not conserved, as there is more afterwards than before. You can still say total energy was conserved, as all this new translational energy was released from the gunpowder's chemical energy, but translational kinetic energy by itself is not conserved.)

"I am somewhat confused about momentum. If impulse is equal to mass times the change in velocity, is momentum simply the change in velocity?" (Momentum is mass times velocity. Assuming that mass remains constant, then the change in momentum is the mass times the change in velocity.)

Online reading assignment: collisions

Physics 205A, fall semester 2015
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.

"In an elastic collision, translational kinetic energy is conserved. In completely inelastic and partially inelastic collisions, translational kinetic Energy is not conserved; instead, it is used in the deformation of the objects as they interact."

"Completely elastic means the two objects rebound off each other with no damage and no sticking together, completely inelastic is where the two objects stick together with damage, and in-between is partially inelastic which means there is some damage, but they didn't stick together afterwards."

"Momentum, like energy, must be conserved so p_f must be equal to p₀ but it is the velocities that change for the masses within the system. Elastic collisions retain the total translational kinetic energy because they are like a rubber band bouncing back. Inelastic collisions would be where the rubber band snaps or becomes too stretched out, and is permanently deformed."

"When the net external forces in a collision are effectively zero, then the system of objects is considered to be a 'closed system,' and the principle of conservation of linear momentum applies, where the total initial momentum will be equal to the total final momentum."

"Internal forces are the forces that the objects within the system exert on each other, and external forces are forces exerted on the objects by agents external to the system."

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.

"Everything seemed to make sense."

"Once we get into elastic and inelastic collisions I will need some review."

"Can't tell how a collision is elastic, partially inelastic, or completely inelastic. Let alone the two kinds of inelastic."

"Everything is confusing, please thoroughly explain this chapter in lecture."

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.

"Two cars stick to each other in a completely inelastic collision, then they convert as much kinetic energy as they can into permanent deformation, so kinetic energy is definitely not conserved. If two cars deform and then rebound in a (partially) inelastic collision, then some kinetic energy goes into permanent deformation, while some of it is converted back to into kinetic energy (such that the cars will rebound off each other), so kinetic energy is also not conserved."

Partially inelastic collision has objects that don't stick together, and completely inelastic collision has objects that do stick together."

"The difference is whether or not the two objects stick to each other, or not, and how much translational 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.

"Because the time is so small that the friction is negligent."

"A collision is so short in duration that other forces are insignificant during this time."

"not enough time elapsed for those forces to act upon brief collisions."

"During the very short duration of a collision the net force effect of drag, friction, etc. are not important."

"I don't remember."

"Collisions occur in fractions of seconds. Such a small amount of time makes the impulse of external forces negligible."

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.	**** [4]
(Partially) inelastic.	******************************************************* [55]
Elastic.	*** [3]
(Unsure/lost/guessing/help!)	* [1]

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.	*** [3]
(Partially) inelastic.	************ [12]
Elastic.	*********************************************** [47]
(Unsure/lost/guessing/help!)	* [1]

For the train and minivan crash, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	************************************************ [48]
(Partially) inelastic.	******* [7]
Elastic.	****** [6]
(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.	**************************************** [40]
Elastic.	*********** [11]
(Unsure/lost/guessing/help!)	******* [7]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)

Completely inelastic.	*********** [11]
(Partially) inelastic.	******************** [20]
Elastic.	************************* [25]
(Unsure/lost/guessing/help!)	******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Excited for the next midterm to prove my understanding."

"How can you tell in the case of the bullet in the gun if it's elastic or not?" (Definitely not elastic, because translational kinetic energy is definitely not conserved. Turns out it's completely inelastic--if you run the process in reverse, a slowly moving rifle collides with a bullet quickly moving into its barrel, and both stick together. So whether you let time run backwards or forwards, it's still completely inelastic.)

"The book states that when dealing with subatomic particles translational kinetic energy is often conserved. Why is that?" (At that level there is no where else for energy to be lost--friction, drag, etc., don't matter to subatomic particles, so they just keep moving along.)

"Your blog saves my life. Screw the textbook."

"How long does it take you to prepare your online blog presentations?" (Each presentation takes a whole week to start from scratch, most of them were done over the past few summers.)

"You might not believe it, some people still don't read through your blog posts and presentation previews. THESE ARE SO HELPFUL. Please remind the class that they exist, as lecture is very helpful after reading them, and nearly useless without."

"I really want to go skydiving and a get a tattoo. I also want my tattoo to be a tramp stamp. #hatersgonnahate" (#yolo)

"Are you doing anything for Halloween?" (My people celebrate Madonnaween.)