Final answer:
An ordinary rifle recoils when fired because of Newton's third law of motion. A recoilless rifle reduces recoil by allowing gases to exit from the rear, but it is not safe to stand close behind one due to the high-velocity gases ejected.
Step-by-step explanation:
Understanding Rifle Recoil and Recoilless Rifles
When an ordinary rifle is fired, it recoils or kicks backward due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. The explosion propelling the bullet forward also pushes the gun backward. In contrast, a recoilless rifle, which is open at both ends, allows gases generated by the firing to exit from the rear, counteracting the forward momentum of the bullet and reducing recoil significantly.
Newton's third law applies to a recoilless rifle in that the force propelling the bullet forward is balanced by the force of the gases exiting out the back, resulting in no net recoil. However, standing close behind a recoilless rifle when it is fired is extremely dangerous because the high-velocity gases ejecting from the rear can cause injury. Hence, a recoilless rifle should not be misconstrued as safe to stand behind during discharge.