Final answer:
When a rifle is fired, it recoils due to the principle of Newton's third law of motion. A recoilless rifle, with an open barrel at both ends, does not experience significant recoil. It is not safe to stand close behind a fired rifle.
Step-by-step explanation:
When a rifle is fired, it experiences a recoil (kick backward) due to a principle known as Newton's third law of motion. According to this law, for every action, there is an equal and opposite reaction. When the bullet is fired forward with a force, the rifle experiences an equal and opposite force pushing it backward, resulting in recoil.
A recoilless rifle, on the other hand, has an open barrel at both ends. When it is fired, the high-pressure gas escapes from both ends simultaneously, canceling out the forward and backward forces. Therefore, the rifle does not experience any significant recoil.
It is not safe to stand close behind a rifle when it is fired, regardless of whether it is an ordinary rifle or a recoilless rifle. The high-pressure gases and debris expelled from the barrel can cause serious injury or even death. It is important to maintain a safe distance and follow proper gun safety protocols.