Final answer:
An ordinary rifle recoils when fired due to Newton's third law of motion, where the action of the bullet being propelled forward results in an equal and opposite reaction force that pushes the rifle backward. This is distinctly different from the principles of conservation of energy and momentum that govern system behavior but are not the direct cause of recoil. The correct answer is (c) Newton's third law.
Step-by-step explanation:
The ordinary rifle recoils or 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 gunpowder ignites, causing an explosion that propels the bullet forward. Simultaneously, a backward force is exerted on the rifle. This backward force is the reaction to the forward force on the bullet, causing the rifle to recoil.
A recoilless rifle, however, is designed with an open barrel at both ends. When this type of rifle is fired, gases produced by the ignition of gunpowder are expelled out the rear as well as the front. By doing this, the forces on the front and back of the rifle cancel each other out, greatly reducing or nearly eliminating recoil. It is not safe to stand close behind a recoilless rifle because the high-velocity gases and possible debris exiting from the rear can pose a significant danger.
Conservation of energy and conservation of momentum are also involved in the process, as they govern the behavior of the rifle and bullet system and ensure that energy and momentum are conserved before and after the bullet is fired. However, when specifically discussing the cause of recoil, Newton's third law is the correct explanation. Friction with the air has negligible effect on the rifle's recoil.
Therefore, the correct option for why an ordinary rifle recoils when fired is (c) Newton's third law.