Elastic Collisions

[PreMedMachine]

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Which of the following are elastic collisions?
I. A planet breaks into several fragments.
2. Two balls collide and then move away from each other with the same speed but reverse directionality.
III. A volleyball hits the net, slows down, and then bounces back with one-half its initial speed.
A I only
B II only
C II and III only
D I, II, and III


The answer is B, II only, but can someone please explain what types of collisions I, II, and III are as well as when momentum and KE are conserved/not conserved? Thanks 🙂))!!!

 

[chill3]

II. Since KE is conserved it is elastic collision

III. You can imagine a large portion of total E being transformed into deformation of the net, therefore this is a perfect example of inelastic collision

I. I don't really see where the collision is in a planet spontaneously breaking apart, but if something did hit it and it broke apart then some amount of E is required to break apart the bonds holding the planet together and therefore KE is not conserved. Inelastic.

Basically if KE is conserved its elastic. This is rare in real world macroscopic collisions and more common in scenarios where the two (or more) objects do not physically touch. However, for simplicity many questions will define the system as elastic, as done in answer II.

 

[Blueprint MCAT Tutor]

About I -- we're given no info about the motion of the original planet and the motion of the subsequent pieces. If the planet were just sitting there and then suddenly "blew up" (a la Krypton) then we could say the planet initially had no KE since it wasn't moving and then the pieces had lots of KE as they flew away. Really, it's an oddly phrased choice and doesn't sound quite MCAT-like.

As chill3 noted above, nearly everything is inelastic. If you want a system to have elastic collisions you've got to have a reason for thinking it - either the question explicitly says they're elastic, or says it's an ideal gas (with elastic collisions bwn molecules), or it says they have the same speed (and hence same KE) after the collision or something.