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# I cant get my head around this M2 collisions problem watch

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1. A(m) is going at 3u

B(2m) is going at -u

A is reversed in collision

coefficeint of restitution is e

find v2 ( i got 1/3u(1+4e))

show that e>1/8 ??? i cant get this

some1 help me
2. Find what e must be to stop A dead. e must be greater than this if motion of A is to be reversed
3. i still dont get 1/8?
4. Can you show your working how you got to to v2 = 1/3u(1+4e)?

Also, can you clear that up a bit, is it 1/(3u(1+4e)) or (1+4e)/(3u)?

Edit: make sure you make B's momentum opposite in direction to A's (opposite sign) and that B is twice the mass of A
5. well part a of the question said

show u(1+4e)1/3 is B's vel after collions (sorry if unclear b4)
6. And you have worked this out?

All you need for this question is conservation of momentum: 3mu-2mu=mv1 + 2mv2

taking A's initial velocity as positive. v1 is A's final velocity, and v2 is B's final velocity

and e=(speed of separation)/(speed of approach) = 4u / (v2 - v1)

v1 and v2 are related by u, so finding one will give you the other. This leaves you with 2 unknowns (e and v1 or v2) and 2 simultaneous equations to do this by.

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Updated: October 16, 2006
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