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# Physics A level momentum question watch

1. A moving ball of mass M and speed v collides head-on with a stationary ball of different mass.
1) After the collision, the first ball is stationary and 10% of the kinetic energy is lost. Show that the mass of the second ball is 10M/9.
Struggling to prove this, can anyone show a little bit of guidance would be much appreciated.
2. conservation of momentum:

m1
v1 + 0 = 0 + m2v2

KE after = 0.9 KE before...

1/2m2 v22 = 0.9*1/2m1 v12

now you can find the answer.

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