Q 11-04-018NEETJEE MainEAMCET 2007 (Engineering)Easy
A rifle of $20$ kg mass can fire $4$ bullets per second. The mass of each bullet is $35 \times 10^{-3}$ kg and its final velocity is $400\ \text{m s}^{-1}$. Then, what force must be applied on the rifle so that it does not move backwards while firing the bullets?
Answer: (D) $56$ N
The force equals the rate at which momentum is given to the bullets:
$$F = n\,mv = 4 \times 35 \times 10^{-3} \times 400 = 56\ \text{N}$$
(The mass of the rifle is not needed.)
Solution by Sreeraj P, M.Sc Physics