Is the momentum of a system conserved even with friction?

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Discussion Overview

The discussion revolves around the conservation of momentum in a system that includes friction, particularly in the context of collisions. Participants explore whether friction should be considered an external force and how this affects momentum conservation during and after collisions.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants suggest that the frictional force may not be an external force, allowing for momentum conservation if measured correctly before and after a collision.
  • Others argue that the classification of friction as internal or external depends on the definition of the system, specifically whether the floor is included.
  • One participant emphasizes the need for precise description of the system and the specific frictional forces involved.
  • It is noted that during the brief duration of a collision, friction has minimal effect on momentum conservation, but it influences the speeds of objects over larger time scales.
  • Another participant points out that if the floor is included in the system, friction becomes internal, complicating the application of momentum conservation.
  • Some participants express that the nature of friction is similar to other forces acting between an object and the floor, with no unique characteristics that set it apart in this context.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether friction is an external force, as opinions vary based on how the system is defined. Multiple competing views remain regarding the implications of including or excluding the floor in the system.

Contextual Notes

The discussion highlights the dependence on definitions of the system and the assumptions made about the forces involved, particularly regarding the treatment of friction in momentum conservation scenarios.

cozycoz
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It's really confusing if the frictional force IS an external force..
My guess is the frictional force isn't an external force
and therefore I can observe the momentum conservation even with friction if I carefully measure the velocity right before and after the collision.
But I'm not sure about it..
 
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cozycoz said:
It's really confusing if the frictional force IS an external force..
My guess is the frictional force isn't an external force
and therefore I can observe the momentum conservation even with friction if I carefully measure the velocity right before and after the collision.
But I'm not sure about it..
It depends on what your system is: does it include both objects exchanging the friction force?
 
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It would probably help if you describe your system more precisely, and tell us which frictional force(s) you are concerned about.
 
@russ_watters
@jtbell
Oops yes
It's conventional 2d collision!
And the friction I mentioned is between the floor and objects.
 
cozycoz said:
And the friction I mentioned is between the floor and objects.
In terms of momentum, it's no different than the normal contact force.
 
cozycoz said:
And the friction I mentioned is between the floor and objects.
Aha. :cool: I suspected that might be the case because of your comment
cozycoz said:
even with friction if I carefully measure the velocity right before and after the collision.
but I wanted to make sure. This friction has very little effect during the very short time duration of the collision itself, so it doesn't affect (for practical purposes) conservation of momentum immediately after versus immediately before the collision. However, it does affect the speed of the objects at larger times after and before the collision. Therefore, in order to best test conservation of momentum, you need to measure the speeds as close to the collision as you can.

Added: I'm considering the system to consist of only the two colliding objects, so the friction from the floor is external. If you include the floor (earth) in the system, the friction becomes internal and you maintain conservation of momentum at all times, but now you have a three-body problem to solve, when applying conservation of momentum!
 
Last edited:
cozycoz said:
My guess is the frictional force isn't an external force
That's the thing; sometimes it's internal and sometimes it's external,, depending. You have clearly looked for a reason to bring your intuition in line with a very basic 'Law' and that's very healthy. Looking for reasons for being wrong is far better than looking for reasons why Physics is wrong - and we get an amazing number of posts about the latter!
This is not to say that Physics can't advance; it's just that you can only make that happen when you have eliminated all possibilities of yourself being wrong. (A la Sherlock Holmes)
 
Is friction force between floor and a object in system external force or not depends whether floor is part of system or not.
Probably it is easier not to consider floor as part of system,and consider friction force between floor and a object in system an external force.
 
olgerm said:
Is friction force between floor and a object in system external force or not depends whether floor is part of system or not.
And the same is true for any other force between an object and the floor. There is nothing special about friction in that regard.
 

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