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Recoil of a gun
Beginner
Bullet and gun gain equal and opposite momenta.
M V = −m vA gun and bullet start at rest, so total momentum stays zero: MV = −mv. The gun's recoil speed is small because its mass is large.
0.02 kg bullet at 400 m/s, 4 kg gun: recoil 2 m/s.
The gun and bullet get equal and opposite momentum but very different kinetic energy.
In more detail
When a gun fires, the bullet and the gun receive equal and opposite momenta. The gun recoils backwards because its mass is larger, so its speed is smaller.
Real-world uses
- Engineers design gun mounts and stocks to absorb recoil.
- Fireworks rockets use recoil from expelled gases to move forward.
- A cannon mounted on a cart recoils backward when it fires, because the total momentum of the cannon and shell is initially zero.
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Frequently asked questions
Why does a heavy gun recoil less?
For the same momentum, a larger mass gives a smaller velocity.
Is recoil the same as the reaction force?
Yes. It is the reaction to the force pushing the bullet forward, following Newton's third law.
Why is the recoil velocity smaller for a heavier gun?
The gun's larger mass shares the same momentum, so its velocity is reduced in proportion to its mass.
Related topics in Momentum & Collisions
Why airbags work · Conservation of momentum · Isolated system · Elastic collision · Inelastic collision · Perfectly inelastic collision