Two balls are projected vertically upward from the same point with the same speed, the second ball being projected 2 s after the first. Taking g=10 m s⁻², after the second ball is projected the magnitude of their relative velocity is:
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Question

Two balls are projected vertically upward from the same point with the same speed, the second ball being projected 2 s after the first. Taking g=10 m s⁻², after the second ball is projected the magnitude of their relative velocity is:

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G1 Physics · Accepted Answer

Both balls have the same downward acceleration g. Hence their relative acceleration is zero. At the instant the second ball is projected, the first ball has velocity v₁ = u−g(2), while the second has velocity u. Relative velocity magnitude is therefore |u−(u−20)| = 20 m s⁻¹. It remains constant while both are in flight.