Question

A disc of mass $5kg$ is kept floating horizontally in mid air by firing $10$ bullets per second vertically up. If the mass of each bullet is $50gm$ and bullets rebound with same speed, the speed of each bullets is $(g=10{ms}^2)$

• A. $100{ms}^{-1}$
• B. $200{ms}^{-1}$
• C. $50{ms}^{-1}$
• D. $10{ms}^{-1}$

Answer 1

The correct option is A $100{ms}^{-1}$

Disc floating in air by firing $10$bullets per sec.The net force action above the disc is balanced their weight.

Change in momentom per second=$10\times mv$

$F\times 1=10\times 50\times v\times {10}^{-3}$

Where $v$ is velocity of bullet fired.

$F=Mg$

$v=\frac{5\times 10}{0.5}=100$m/s