Stone A is dropped from rest from the top of a tower. Exactly 2 s later stone B is projected vertically downward from the same point at 30 m/s. Take g=10 m/s². How long after B is projected does B catch A?
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Question

Stone A is dropped from rest from the top of a tower. Exactly 2 s later stone B is projected vertically downward from the same point at 30 m/s. Take g=10 m/s². How long after B is projected does B catch A?

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G1 Physics · Accepted Answer

At B's launch, A is 20 m below and moving downward at 20 m/s. Both subsequently have acceleration g, so relative acceleration is zero and B gains at 30−20=10 m/s. Catch time=20/10=2 s.