With what minimum horizontal velocity ‘u’, (in m/s) can a boy throw a rock at A so that it just clear the obstruction at B?

Question

G1 Physics · Accepted Answer

The rock is thrown horizontally from A, 36 m above the ground. The top of obstruction B is 16 m high and 40 m horizontally away, so the required vertical drop at B is 20 m. If t is the time to reach B, 20 = ½gt² = 5t², so t = 2 s. Hence u = 40/2 = 20 m/s. Therefore the answer is 20.