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# M2 Question watch

1. Check out the attachment. I can pretty much do b) providing I got a), but my answer completely differs from a).

Just so that u can do a) nice and quickly: take conservation of momentum to get V in terms of the two unknowns. Then use coefficient of restitution e = v/u and do a bit of substitution to get the two unknown speeds in terms of e and V.

Velocity of A (the ball with mass m) = V[1/3 - 2/(3e)]
Velocity of B (the ball with mass 2m) = V/(3e) x [(1+e)]

The correct answers in the book are:
Velocity of A (the ball with mass m) = 1/3(1-2e)V
Velocity of B (the ball with mass 2m) = 1/3(1+e)V

I struggle to see why 'e' is not part of a denominator of a fraction for either velocities, and therefore getting a wrong answer for part a) and b)! Please help!
Attached Images

2. (Original post by ResidentEvil)
Check out the attachment. I can pretty much do b) providing I got a), but my answer completely differs from a).

Just so that u can do a) nice and quickly: take conservation of momentum to get V in terms of the two unknowns. Then use coefficient of restitution e = v/u and do a bit of substitution to get the two unknown speeds in terms of e and V.

Velocity of A (the ball with mass m) = V[1/3 - 2/(3e)]
Velocity of B (the ball with mass 2m) = V/(3e) x [(1+e)]

The correct answers in the book are:
Velocity of A (the ball with mass m) = 1/3(1-2e)V
Velocity of B (the ball with mass 2m) = 1/3(1+e)V

I struggle to see why 'e' is not part of a denominator of a fraction for either velocities, and therefore getting a wrong answer for part a) and b)! Please help!
Haven't done m2 for a while...but isn't the coefficient of restitution:

e = (Vb-Va)/(Ua-Ub) ????????

v=eu is only for collisions with a surface...like a wall..

G
3. (Original post by gzftan)
Haven't done m2 for a while...but isn't the coefficient of restitution:

e = (Vb-Va)/(Ua-Ub) ????????

v=eu is only for collisions with a surface...like a wall..

G
yeah but i just said "e=v/u" as a generalised form which is what its based on.
In this situation u do do
e = (Vb-Va)/(Ua-Ub) depending if those are the directions taken
4. Ok...i've only done Vb..but maybe that'll set you on your way with the question...

CLM: mV = mVa + 2mVb => V = Va+2Vb (1)

using e....

e = (Vb-Va)/V

Ve = Vb-Va => Va = Vb - Ve (2)

Sub (2) into (1)

V = (Vb-Ve)+2Vb

V + Ve = 3Vb

V(1+e) = 3Vb

1/3(1+e)V = Vb

Hope this helps

G
5. duhhhh I AM SO STUPID!!!!. Thanks mate
6. (Original post by ResidentEvil)
duhhhh I AM SO STUPID!!!!. Thanks mate
No probs

G

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