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# M2 momentum/ restitution help

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1. Q7i

First of all can someone explain what it means if the speed of A>B after the collision?
Also how do I know which directions A and B are moving after the collision?
Am I assuming C is stationary throughout the collision
Then I tried setting up simultaneous equations.

I let V1 the speed of A after and V2 the speed of B after but I wasn't sure on the directions.

Thanks
https://9565af48d144678aee5dbc2a15d4...20M2%20OCR.pdf
2. (Original post by Super199)
Q7i

First of all can someone explain what it means if the speed of A>B after the collision?
It just means that if the order is

[A] ------- [B]

then if v(A) > v(B) the particle A will "catch up" to the particle B and collide.

If the order is

[B] ------ [A]

then v(A) > v(B) means that B will never catch up to B and they won't collide. It's the former scenario, I suspect - but just writing this here for other scenarios.

Also how do I know which directions A and B are moving after the collision?
At the beginning you should have taken one direction as positive. Then the values of your velocity will tell youthe direction, if the velocity is positive then it's going in the direction you originally took to be positive and if it's negative then it's the opposite direction to the direction you originally took to be positive.
3. (Original post by Zacken)
It just means that if the order is

[A] ------- [B]

then if v(A) > v(B) the particle A will "catch up" to the particle B and collide.

If the order is

[B] ------ [A]

then v(A) > v(B) means that B will never catch up to B and they won't collide. It's the former scenario, I suspect - but just writing this here for other scenarios.

At the beginning you should have taken one direction as positive. Then the values of your velocity will tell youthe direction, if the velocity is positive then it's going in the direction you originally took to be positive and if it's negative then it's the opposite direction to the direction you originally took to be positive.
But if I don't know the velocities after. Then what?
4. (Original post by Super199)
But if I don't know the velocities after. Then what?
What do you mean? Set them as Va and Vb and put them in any direction you want, it doesn't matter. If you picked the wrong direction it'll be a negative value, if you picked the right direction, it'll be a positive value.

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