Question

A gun recoils backward after firing a bullet:
A) To follow the law of inertia
B) To conserve momentum
C) To conserve mass
D) The statement is wrong

Answer 1

Hint: It is a true statement that a gun cannot be used without proper body training. The question is why is it necessary to train the muscles in the hand for high strength when they use the gun? Also, why is it necessary to mount very huge guns such as high precision sniper rifles, rocket launchers on the shoulders and supporting it with both the hands?
The one answer to the above two questions is recoil and it is explained in the answer here.

Complete step by step answer:
Whenever a bullet is shot from the gun, the hand holding the gun experiences a strong backward jerk after the bullet leaves the gun. This phenomenon is called recoil.
Let us understand how the recoil takes place.
Assume a gun where the mass of its bullet is $m$ and it travels at a velocity $v$. When this bullet is fired, it acquires a huge change in its momentum from its state of rest to a very huge velocity.
Momentum acquired by the bullet, $p = mv$
Now, the question arises as to how the momentum of the bullet rises to such high values within seconds?
The answer is that the momentum does not get created out of nowhere. The momentum is conserved due to the law of conservation of momentum.
This huge increase in momentum has a consequence to bear and it is called the recoil. When the momentum of the bullet picks up to some huge value, the same momentum is realised on our hands since the total momentum of the system is equal.
The momentum experienced by the hand is P. If M is the mass of our hand and V is the recoil velocity,
$p = P$
$mv = MV$
$\Rightarrow V = \dfrac{{mv}}{M}$
This is the equation to calculate the recoil velocity.
As we can see, the recoil of the gun happens due to the law of conservation of momentum.

Hence, the correct option is Option (B).

Note: The main disadvantage of recoil velocity is that it results in error and inaccuracy in shooting at the right target because of the jerky motion created by the hand after shooting. This answer explains the fact as to why the hand must be sufficiently strong to handle a gun.
Recoil velocity, $V \propto \dfrac{1}{M}$
The recoil velocity, V is inversely proportional to mass of the hand M. It means that more the muscular hard mass in the hand, the lesser is the recoil velocity experienced by the hand and more accuracy in targeting.