Question

A disc of mass $100\, g$ is kept floating horizontally in air by firing bullets, each of mass $5\, g$ with the same velocity at the same rate of $10$ bullets per second. The bullets rebound with the same speed in opposite direction, the velocity of each bullet at the time of impact is:

• A. 196 cm/s
• B. 9.8 cm/s
• C. 98 cm/s
• D. 980 cm/s

Answer 1

Answer: (D)

980 cm/s

Answer 2

From law of conservation of momentum Total momentum of bullets $=$ Momentum of disc $2 m' \eta n=m g$ $v=\frac{m g}{2 m' n} =\frac{100 \times 980}{2 \times 5 \times 10}$ $=980 \,cm / s$