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 $cms^{-1}$
• B. 9.8 $cms^{-1}$
• C. 98 $cms^{-1}$
• D. 980 $cms^{-1}$

Answer 1

Answer: (D)

980 $cms^{-1}$

Answer 2

From law of conservation of momentum. Total momentum of bullets= Momentum of disc ie, $\, \, \, \, \, \, \, \, \, \,$ 2m' vn = mg $\therefore \, \, \, \, \, v = \frac{mg}{2m'n} = \frac{100 \times 980}{2 \times 5 \times 10}$ $\, \, \, \, \, \, = 980 cm s^{-1}$