Recoil
Definition
Recoil is the backward kick you feel when something shoots forward — a gun firing a bullet, a balloon releasing air, or a rocket launching.
Technical definition
A direct consequence of momentum conservation: in an isolated system the total momentum stays zero, so the forward momentum of the projectile equals the backward momentum of the launcher (m₁v₁ = m₂v₂).
Example
A 0.01 kg bullet leaving a 4 kg rifle at 800 m/s gives the rifle a recoil speed of (0.01×800)/4 = 2 m/s — a sharp but manageable kick into the shooter's shoulder.
Common confusion
Recoil is not caused by an explosion pushing on the air behind the gun. It happens even in a vacuum: the gun pushes the bullet forward, the bullet pushes the gun backward — Newton's third law, with no air needed.