Impulse is equal for rebounding projectiles, even when speed changes?

[hellocubed]

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I am having a really hard time understanding this concept.

In a head-on collision between 2 identical balls each traveling at 20 m/s in opposite directions results in both rebounding with a speed of 10 m/s.
BR says here that they both have the same magnitudes of momentum before and after, so impulse in conserved.


I was wondering if anyone could explain what this means to me, because they clearly have very different magnitudes of momentum before and after the collision on an order of x2. Also, what does it mean that Impulse "conserved? Impulse is a value of change in momentum... And considering momentum changes, impulse clearly is not 0.

 

[syoung]

you are correct, F1T = -F2T (equal and opposite impulses) as well as m1v1=-m2v2 are conserved especially in head-on collisions. Remember impulse is Ft and it equals mv --> Ft = mv
but i'm not sure how they rebound with 10 m/s each. Unless some energy was lost.

 

[hellocubed]

ahhh, ok

F1T = -F2T


makes sense thanks.
It's an inelastic collision so that it loses energy, but momentum is conserved.

I believe that if momentum is not conserved, E has been added to or left the system?

 

[syoung]

you are correct. Momentum is always conserved in an elastic collision, as is energy and impulse. Once it is inelastic, some of the KE has been lost due to heat.