Experimental questions about the law of momentum

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Discussion Overview

The discussion revolves around the application of the law of momentum conservation in a scenario involving two carts, one equipped with a plunger. Participants explore the implications of momentum and kinetic energy before and after the release of the plunger, questioning the nature of the collision and the conservation principles involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how momentum conservation applies when both carts start from rest, leading to an initial momentum of zero and a non-zero momentum after the plunger is released.
  • Another participant clarifies that momentum is a vector quantity, suggesting that opposing momenta can sum to zero.
  • A participant asks whether the collision is elastic or inelastic and discusses the kinetic energy before and after the collision, noting that initial kinetic energy is zero.
  • Another participant asserts that the collision cannot be elastic due to the initial kinetic energy being zero and introduces the concept of work done by the plunger contributing to kinetic energy.
  • A participant summarizes that while momentum conservation holds for all types of collisions, kinetic energy may not be conserved, distinguishing between elastic and inelastic collisions based on kinetic energy changes.

Areas of Agreement / Disagreement

Participants generally agree that the law of momentum conservation applies regardless of the type of collision, but there is ongoing discussion about the nature of the collision (elastic vs. inelastic) and the conservation of kinetic energy, indicating some disagreement or uncertainty in definitions and implications.

Contextual Notes

Participants have not fully resolved the definitions of elastic and inelastic collisions in the context of the scenario, and there are assumptions regarding the work done by the plunger that remain unexamined.

-EquinoX-
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if say I have two carts that is initially at rest, one of the cart has a plunger and the plunger is set so that it's against the other cart. When I release the plunger both carts will travel at a certain velocity.

The law of momentum conservation says that the momentum of the initial system is the same as the momentum of the final system.

As far as my understanding goes, the momentum before the what we can say "collision" is 0 as both carts has an initial velocity of 0, as they start from rest. While on the other hand the momentum after collision (after I release the plunger) is some number, as the velocity is not 0. So how is it possible that law of momentum holds here?

p initial is not equal to p final

I use p to denote momentum here.

Can someone please help me to clear my understanding
 
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Remember that momentum is a vector quantity. So if one cart has a momentum of 2 kg m/s to the left and if the other cart has a momentum of 2 kg m/s to the right then the total momentum is 0.
 
if you consider at my example here, would you consider this as an elastic or inelastic collision? also how about the kinetic energy before and after the collision?

as far as I understand, before the collision the kinetic energy is 0 as there is no initial velocity
 
-EquinoX- said:
if you consider at my example here, would you consider this as an elastic or inelastic collision? also how about the kinetic energy before and after the collision?

as far as I understand, before the collision the kinetic energy is 0 as there is no initial velocity

Yes, that is correct- so this obviously is NOT an "elastic" collision. In order to conserve energy you would have to consider the work done in pressing in the plunger which turns into kinetic energy when it is released.
 
okay, so on whatever type of collision it is (elastic or inelastic), the law of momentum conservation always holds.. however the kinetic energy before and after the collision is not always the same.. if the kinetic energy of before and after is the same then its elastic collision if not then it's inelastic. Am I right?
 
Right. :smile:
 

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