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# m2 collisions

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1. Can someone help me with part b?

A particle P of mass 3m is moving with speed 2u in a straight line on a smooth horizontal
plane. The particle P collides directly with a particle Q of mass 4m moving on the plane with
speed u in the opposite direction to P. The coefficient of restitution between P and Q is e.
(a) Find the speed of Q immediately after the collision.

Answer: U/7 (9e+2)

Given that the direction of motion of P is reversed by the collision,
(b) find the range of possible values of e.

Would i make the final speed of P >0 or P<0 and please give me a reason why it is the one you have chosen?
2. For P to reverse, it's velocity must change sign, assuming you took P to be travelling right and right to be positive,
Put your speed of Q back into the equation for conservation of momentum in order to find some equation in e and u for the velocity of P, make this velocity <0 and solve for e.
3. (Original post by JN17)
For P to reverse, it's velocity must change sign, assuming you took P to be travelling right and right to be positive,
Put your speed of Q back into the equation for conservation of momentum in order to find some equation in e and u for the velocity of P, make this velocity <0 and solve for e.
That really helped me thank you.

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