Question

A disc of mass 1.0 kg is floating horizontally in air by firing bullets of mass 0.05 kg each vertically at it, at the rate of 10 per second. If the bullets rebound with the same speed, the speed with which these are fired will be-

Answer 1

Weight of disc = mg = 1 kg × 10 m/s² = 10 N

Force exerted by 10 bullets must be equal to 10 N

F = Δp / t
F = nm × (v - u) / t
F = nm × (v - (v)) / t
F = 2nmv/t

From above
v = Ft / (2nm)
= (10 N × 1 s) / (2 × 10 × 50 × 10^-3 kg)
= 10 m/s

Speed of each bullet is 10 m/s