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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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    $\begingroup$ please refine your question to show your previous working so we can help you understand the physics. $\endgroup$ – Emilio Pisanty May 1 '12 at 12:04
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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.

(I did not downvote but your question shows zero own effort which is not appreciated here, as the complete answer is contained in the first google hit to 'elastic collision'.)

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