Is the momentum of a system conserved even with friction?

In summary, the conversation discussed the confusion surrounding whether frictional force is an external force. The consensus was that it depends on the system being considered. If the system includes both objects exchanging the friction force, then it is not an external force. However, if the system only consists of the two colliding objects, then the friction from the floor is considered an external force. This can affect the conservation of momentum, but it can be mitigated by measuring the speeds of the objects closer to the collision. The conversation also highlighted the importance of considering all possibilities and eliminating personal biases in understanding physics.
  • #1
cozycoz
13
1
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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  • #2
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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  • #3
It would probably help if you describe your system more precisely, and tell us which frictional force(s) you are concerned about.
 
  • #4
@russ_watters
@jtbell
Oops yes
It's conventional 2d collision!
And the friction I mentioned is between the floor and objects.
 
  • #5
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.
 
  • #6
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!
 
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  • #7
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)
 
  • #8
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.
 
  • #9
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.
 

Related to Is the momentum of a system conserved even with friction?

1. Is the momentum of a system always conserved?

In an isolated system, where there is no external force acting on the system, the total momentum of the system is always conserved. This means that the initial momentum of the system will be equal to the final momentum of the system, regardless of any internal interactions or collisions.

2. How does friction affect the conservation of momentum?

Friction is a non-conservative force that acts against the motion of an object. This means that when friction is present, some of the energy and momentum of the system is lost to heat and sound. Therefore, the total momentum of the system is not conserved when there is friction present.

3. Can momentum be conserved even with friction?

In some cases, momentum can still be conserved even with the presence of friction. This occurs when the frictional force is equal and opposite to the force causing the motion, resulting in a net force of zero. However, this is not always the case and momentum will not be conserved if there is a net external force present.

4. How does the coefficient of friction affect the conservation of momentum?

The coefficient of friction is a measure of the amount of friction between two surfaces. A higher coefficient of friction means that there is a greater resistance to motion, resulting in more energy and momentum being lost to friction. Therefore, a higher coefficient of friction will have a greater impact on the conservation of momentum.

5. Is the conservation of momentum violated if there is friction?

No, the conservation of momentum is not violated with the presence of friction. While friction may cause some energy and momentum to be lost, the total momentum of the system is still conserved. However, the conservation of energy may be violated due to the energy lost to friction.

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