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 pf 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."
Completely inelastic. **** [4] (Partially) inelastic. ******************************************************* [55] Elastic. *** [3] (Unsure/lost/guessing/help!) * [1]
Completely inelastic. *** [3] (Partially) inelastic. ************ [12] Elastic. *********************************************** [47] (Unsure/lost/guessing/help!) * [1]
Completely inelastic. ************************************************ [48] (Partially) inelastic. ******* [7] Elastic. ****** [6] (Unsure/lost/guessing/help!) ** [2]
Completely inelastic. ***** [5] (Partially) inelastic. **************************************** [40] Elastic. *********** [11] (Unsure/lost/guessing/help!) ******* [7]
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.)
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