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# edexcel M1 impulse watch

1. we have to balls A and B moving in opposite directions.they collide.
A receives impulse from B...to find the impulse from B on A in the formula I=m(v-u) i use the mass and velocities of B right?
2. Thats correct, I = m(v - u) decribes the change in momentum of B, since momentum cannot dissapear in a perfect collision, that change has to have gone somewhere. This is the impulse on A.
3. You use the velocities of A to find the impulse on A by B, also remember impulse has direction.
4. (Original post by Kameo)
You use the velocities of A to find the impulse on A by B, also remember impulse has direction.
i think its the reverse...
fro impulse on A velocities of B since B is exerting the impulse on A...
5. It doesn't matter which you chose. With two balls from Newtons 3rd Law, the impulse will be of the same force and for the same time.

That's how you can say m1v1 -m1u1 = m2v2 - m2u2

But yeah, do watch the direction..

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Updated: January 15, 2010
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