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Elastic and inelstic collisions conceptual questions.

  • #1
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Homework Statement


The questions showed in the pictures ask me whether the collisions in the drawings could be elastic or inelastic, I am not given any mass, the angles are a little vague but I think B and C are supposed to be π/2 and π respectively. For velocity, I am just given the direction and somewhat the magnitude but no numbers. Also the dot represents where the particles collided.

Homework Equations


Conservation of momentum: P_initial = P_final
Conservation of energy: KE initial = KE energy final + Q
Q is just the energy lost or gained during the collision.

The Attempt at a Solution


So far I think that all of them could be elastic and inelastic as without any information of the initial state of the system I can't know whether we are loosing some kinetic energy or not. The problem is I'm not sure whether my reasoning is correct as I am not sure how can I prove this. Any help to reason out the correct answer would be very appreciated.
 

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  • #2
haruspex
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There is a very easy way to think about 1d: "backwards".
 
  • #3
haruspex
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1f is somewhat problematic. A collision is inelastic if any KE is lost, no matter how little. Consequently, if a diagram depicts an elastic collision, the same diagram could work for inelastic, however slightly.
My guess is that for 1f you are supposed to consider whether it could have been "perfectly inelastic". That means, as much KE was lost as is consistent with conservation of momentum, no friction, point masses (so no rotation) and non-penetration (they cannot pass through each other).
 
  • #4
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There is a very easy way to think about 1d: "backwards".
Sorry I'm not sure what you mean by working it backwards. Could you explain a little more please
 
  • #5
haruspex
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Sorry I'm not sure what you mean by working it backwards. Could you explain a little more please
A feature of elastic collisions is no change in entropy. The process is in principle reversible, i.e., you can run time backwards to recreate the initial condition. Just reverse the arrows. What does it remain to check?
 
  • #6
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A feature of elastic collisions is no change in entropy. The process is in principle reversible, i.e., you can run time backwards to recreate the initial condition. Just reverse the arrows. What does it remain to check?
Sorry for late reply, Thanks for the help !
 
  • #7
haruspex
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Sorry for late reply, Thanks for the help !
Does this mean you have answered the question?
Were you able to answer my question at the end of post #5?
 

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