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A ball A of mass m falls under gravity from a height h and strikes another ball B of mass m which is supported at rest on a spring of stiffness k. Assume perfect elastic impact. Immediately after the impact
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- the velocity of ball A is 1/2 √2gh
- the velocity of ball A is zero
- the velocity of both balls is 1/2√2gh
- None of the above
Correct Option: B
In a perfectly elastic collision between equal masses of two bodies, velocities exchange on impact.
Velocity just before impect = Velocity immediately after impact. 
