# A rifle of 20 kg mass can fire 4 bullets per second. The mass of each bullets per second. The mass of each bullet is $35 \times 10^{-3}\;kg$ and its final velocity is $400\;ms^{-1}.$ Then what force must be applied on the rifle so that it does not move backwards while firing the bullets?

$\begin {array} {1 1} (a)\;80\;N & \quad (b)\;28\;N \\ (c)\;-112\;N & \quad (d)\;-56\;N \end {array}$

(d) -56 N