Final answer:
The recoil of a cannon when firing is due to Newton's third law, with the forward force on the shell equaled by a backward force on the cannon. Recoilless rifles reduce recoil by the rearward ejection of gases, but standing behind one when fired is unsafe.
Step-by-step explanation:
A cannon recoils when it fires a shell due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When the cannon fires, the explosive force propelling the shell forward is equal and opposite to the force pushing the cannon backward, resulting in recoil. In the case of a recoilless rifle, the shell's expulsion creates a counteracting force by ejecting gases out the back, which reduces recoil. However, it is not safe to stand close behind a recoilless rifle when it is fired, as the high-speed ejecta can be as lethal as the projectile itself.