# B Is the momentum of a system conserved even with friction?

1. May 11, 2018

### cozycoz

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..

2. May 11, 2018

### Staff: Mentor

It depends on what your system is: does it include both objects exchanging the friction force?

3. May 11, 2018

### Staff: Mentor

It would probably help if you describe your system more precisely, and tell us which frictional force(s) you are concerned about.

4. May 11, 2018

### cozycoz

@russ_watters
@jtbell
Oops yes
It's conventional 2d collision!
And the friction I mentioned is between the floor and objects.

5. May 12, 2018

### A.T.

In terms of momentum, it's no different than the normal contact force.

6. May 12, 2018

### Staff: Mentor

Aha. I suspected that might be the case because of your comment
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: May 12, 2018
7. May 12, 2018

### sophiecentaur

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. May 12, 2018

### olgerm

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. May 13, 2018

### A.T.

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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