JEE Main Physics question
A solid disc of mass M is just held in air horizontally by throwing 40 stones per second vertically upwards to strike the disc, each with a velocity 6 ms⁻¹. If the mass of each stone is 0.05 kg, what is the mass of the disc (g = 10 ms⁻²)
Answer: (A)
Solution
For the disc to be held in equilibrium, Mg = n·m·v = 40×0.05×6 = 12 N ⟹ M = 12/10 = 1.2 kg.