A ball `A` is thrown up vertically with a speed `u` and at the same instant another ball `B` is released from a height `h`. At time `t`, the speed `A` relative to `B` is

Options

A. u
B. u - 2gt
C. `sqrt (u^(2) - 2gh)`
D. u - gt

Answer

A

Explanation

Taking upwards motion of ball A for time t, its velocity is `v_A = u - gt`.
Taking downwards motion of ball B for time t, its velocity is `v_B = gt`.
`therefore` Relative velocity of A w.r.t B
=`v_AB = v_A - (-v_B) = (u - gt)-(-gt) = u `