Final answer:
The recoil of a handgun when fired by a policeman is due to Newton's third law, where the action of the bullet being fired forward causes an equal and opposite reaction force that pushes the gun backward.
Step-by-step explanation:
The policeman's handgun recoils slightly during target practice due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When the policeman fires his weapon, the bullet is propelled forward with a certain force, and in response, an opposing force is exerted backward on the gun, causing it to recoil slightly. This backward force on the gun is the reaction to the forward force exerted on the bullet. In military rifles, this recoil is often reduced by mechanisms that allow internal parts like a plunger to recoil over a larger distance, which decreases the average force needed to stop these parts, making the weapon more comfortable to fire.