Question

A disc of mass $10 \, g$ is kept floating horizontally in the 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 positive direction. The velocity of each bullet at the time of impact is $\left(\right.\text{Take }g=9.8 \, m \, s^{- 2}\left.\right)$

• A. $196 \, cm \, s^{- 1}$
• B. 98 $cm \, s^{- 1}$
• C. 49 $cm \, s^{- 1}$
• D. 392 $cm \, s^{- 1}$

Answer 1

Answer: (B)

$2mnv=Mg$
$v=\frac{M g}{2 m n}$
$\Rightarrow u$ $=\frac{10 \times 980}{2 \times 5 \times 10}cm \, s^{- 1}$
$=\frac{9800}{100}cm \, s^{- 1}$
$=98 \, cm \, s^{- 1}$