A bullet of mass 20 g is fired from a rifle of mass 10 kg with a velocity of 100 m/s. The magnitude of the velocity of the recoil of the rifle is:

A bullet of mass 20 gm is fired from a rifle of mass 8 kg with a velocity of 100 m/s. The magnitude of the velocity of the recoil of the rifle is 0.25 m/s.

mass of bullet $m_1=20gm={\displaystyle \frac{20}{1000}}kg=0.020kg$

The velocity of the bullet $v_1=100m/s$

mass of rifle $m_2=10kg$

Let the velocity of the recoil of the rifle be $v$

Using the law of momentum conservation, 

 $m_1{u}_1+m_2{u}_2=m_1{v}_1+m_2{v}_2$

Since initially the bullet and rifle are at rest. 

Hence, $u_1=0,u_2=0$

On substituting the values, we get 
 

$0.020\times 100=10\times -v$ 

V = - 0. 2 m/s

Here. negative sign shows that the direction is opposite to the motion of the bullet.