Question

A Disc of mass 1.0 kg is kept floating horizontally in air by firing bullets of mass 0.05 kg each vertically at it, at the rate of 10 per second. If the bullets rebound with the same speed, the speed with which these are fired will be

• A. $0.098m/s$
• B. $0.98m/s$
• C. $9.8m/s$
• D. $98.0m/s$

Answer 1

The correct option is C $9.8m/s$

Suppose speed of firing is V then after

rebounding the momentum frees fessed

to the dis will be charge in momentum

$\Delta P=-mv-\left(mv\right)=-2mv$

$\uparrow$

initial

Value $=|\Delta P|=2mv$

$\Rightarrow$ Fr 10 bullets $10\times 2=20mv$

$m=0.05kg\Rightarrow |\Delta P{|}_{total}=v$

time $\Delta t=1$ second

$\Rightarrow$ Fore entered $=\frac{|\Delta P{|}_{total}}{\Delta t}=\frac{v}{1}=v$ Newton

it should balance the eight of disc

to prevent it from falling

So $=Mg=1\times 9.8$

$\Rightarrow v=9.8mls$