Final answer:
An ordinary rifle recoils (kicks backward) when fired due to Newton's third law of motion. The same law also applies to a recoilless rifle, although the barrel being open affects the recoil mechanism. It is not safe to stand close behind a recoilless rifle when it is fired due to the strong backward force.
Step-by-step explanation:
An ordinary rifle recoils (kicks backward) when fired due to Newton's third law of motion. According to this law, for every action, there is an equal and opposite reaction. When a bullet is fired from a rifle, the rifle exerts a forward force on the bullet, and in return, the bullet exerts an equal and opposite force on the rifle, causing it to recoil.
When a recoilless rifle is fired, the barrel is open at both ends, allowing the exhaust gases to escape in the opposite direction. Newton's third law still applies in this case. The force exerted by the gases pushing the bullet forward is accompanied by an equal and opposite force pushing the rifle backward, resulting in the recoil.
It is not safe to stand close behind a recoilless rifle when it is fired because the backward force created by the recoil can be quite strong and could cause injury or loss of balance if someone is standing too close.