Final answer:
The recoil experienced when firing a rifle demonstrates the conservation of linear momentum, in accordance with Newton's third law of motion.
Step-by-step explanation:
When someone firing a rifle is pushed back by the rifle, this illustrates that linear momentum is conserved. The law of conservation of momentum states that the momentum of a closed system is constant if no external forces are acting on it. In the case of firing a rifle, the bullet and rifle make up a closed system. When the trigger is pulled, the bullet gains forward momentum, and to conserve momentum, the rifle (and thus the person holding it) is pushed back with equal and opposite momentum. This is an example of Newton's third law, which is often summarized as 'for every action, there is an equal and opposite reaction.'