Question

Two bullets are fired one by a lighter rifle and other by a heavy rifle with the same force .which rifle will hurt the shoulder more and why ?

Answer 1

the recoil of a gun is inversely proportional to its mass ..so as the lighter rifle has lesser mass it'll have a recoil that is greater than the heavy rifle . hence the lighter rifle will hurt the shoulder more.

Answer 2

By Newton's third law, both the rifles will apply same forces on your shoulder as for every action there is an equal and opposite reaction. Hence, the lighter rifle and heavy rifle will hurt equally as the forces applied by them on your shoulder are equal in magnitude. The magnitude of force will measure the amount of hurt on your shoulder. Hence, both the rifles will hurt equally.