# Mathematical question on Collisions [closed]

A 2.5kg ball travelling with a speed of 7.5m/s makes an elastic collision with another ball of 1.5kg and travelling at the speed of 3.0m/s in the same direction.

-- What are the velocities of the balls immediately after collision?

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please refine your question to show your previous working so we can help you understand the physics. –  Emilio Pisanty May 1 '12 at 12:04
In an elastic collision the guiding principles are the conservation of momentum $$p_{1,before} + p_{2,before} = p_{1,after} + p_{2,after}$$ and the similarly the conservation of total kinetic energy. With both you can solve your problem easily.