Conservation of momentum VS friction

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Discussion Overview

The discussion revolves around the effects of friction and conservation of momentum in the context of a train coming to an abrupt stop. Participants explore the implications of a sudden stop on the motion of passengers and the role of friction in deceleration.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether a passenger would hit an object ahead at the same speed as the train due to frictional forces acting on their body.
  • Another participant agrees that friction does slow the passenger down slightly but argues that momentum conservation must be considered, suggesting that the passengers' end velocity would be greater than the train's speed due to the mass difference.
  • A later reply clarifies that if the train stops suddenly by hitting an immovable object, the momentum would be absorbed by that object, and the passenger's motion would continue until acted upon by a force.
  • One participant challenges the earlier claim about momentum conservation, stating that external forces acting on the train mean that momentum is not conserved in the "train + loose objects" system.
  • Another participant acknowledges the oversight regarding the force required to stop the train, indicating a realization of the complexities involved.

Areas of Agreement / Disagreement

Participants express differing views on the conservation of momentum in the scenario described, with some asserting that momentum is conserved while others argue that external forces disrupt this conservation. The discussion remains unresolved regarding the exact implications of these forces on passenger motion.

Contextual Notes

There are assumptions regarding the nature of the stopping force and the definitions of momentum conservation that are not fully explored. The discussion also reflects varying interpretations of Newton's laws in the context of sudden deceleration.

FelBEach
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If you are in a moving train and the train stops instantly would you hit the object ahead of you at the exact same speed the train was traveling or would there be some slight deceleration due to friction? Wouldn't the friction your body experiences with the seat you are sitting on and the air around you cause you to decelerate even at a very minute rate?
 
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You are correct in the point that friction acts to slow the passanger down a little (very little).

However, if a train moves and suddenly stops, it loses its momentum (which has to be conserved at all costs), so that momentum is now shared between every "loose" object in the train (people etc.)

For conservation of momentum we have that

m(train+loose objects)*v.initial=m(loose objects)*v.end

The train has a much larger mass than the loose objects, so the end velocity for them has to be a lot larger than when they were moving with the same speed as the train.

Conclusion, friction slows the passenger down a little, but the passengers total velocity is still larger than when moving at the same speed as the train.
 
when I said the train stops I was thinking the train hit some type of immovable object causing an immediate stop. Wouldn't the momentum from the train be just absorbed by the object and your body would only be moving because it was moving to begin with and just hasn't been stopped yet?
 
Ofey said:
You are correct in the point that friction acts to slow the passanger down a little (very little).

However, if a train moves and suddenly stops, it loses its momentum (which has to be conserved at all costs), so that momentum is now shared between every "loose" object in the train (people etc.)

For conservation of momentum we have that

m(train+loose objects)*v.initial=m(loose objects)*v.end

The train has a much larger mass than the loose objects, so the end velocity for them has to be a lot larger than when they were moving with the same speed as the train.

Conclusion, friction slows the passenger down a little, but the passengers total velocity is still larger than when moving at the same speed as the train.
This is completely incorrect. Momentum of the "train + loose objects" is not conserved--there is an external force acting on the train!

FelBEach said:
when I said the train stops I was thinking the train hit some type of immovable object causing an immediate stop. Wouldn't the momentum from the train be just absorbed by the object and your body would only be moving because it was moving to begin with and just hasn't been stopped yet?
Perfectly correct. Your body will continue moving in a straight line at constant speed unless acted upon by some force. (This is Newton's 1st law.) The seat friction (hopefully) will slow you down a bit before you collide into something.
 
Of course, I am terribly sorry :frown: I was just thinking that the train stopped, never thought about the force making it stop, which obviously has to exist.
 
Thanks, you guys helped me a lot. respect
 

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