# Physics

A ball of mass m makes a head-on elastic collision with a second ball (at rest) and rebounds with a speed equal to one-fourth its original speed. What is the mass of the second ball?

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1. mV=m(-1/4 V)+MV'

1/2 mV^2=1/2m(V^2/16)+1/2MV'^2

form the first equation:
V'=mV(5/4)/M
Put that into the second equation for V', and solve for M

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2. The guy in the top is right !

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3. 5/3m

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