gliderXC t1_j29ugp4 wrote
There are two relevant concepts in physics at play: Momentum and (kinetic) energy.
When you fire the bullet:
- Both the bullet and the shooter get an equal (but opposite) amount of momentum. (see conservation of momentum).
- Momentum = mass x speed. If the mass of the person is 100kg and the bullet weighs 50g the ratio of speed difference between the bullet and the shooter is 50 gr / 100kg*100 = 1/200. This is important for the energy part.
- Both the shooter and bullet get kinetic (movement) energy, but in different amounts. The energy is 0.5 x mass x speed x speed. As per above, the mass and speed of the shooter and bullet are related to each other. As you can calculate, the speed increase of the person is much lower which results in not much energy being transferred to the person.
- In essence, most of energy goes to the bullet since the ratio of weight of the person to the bullet is very large. All kinetic energy is coming from the gun powder (conservation of energy).
- You can imagine receiving the bullet must result in the bullet proof vest to absorb a crazy amount of energy (200 times more than receiving the recoil).
gliderXC t1_j29w727 wrote
To sum it up, when firing a bullet (on a slippery surface):
- Both the bullet and the shooter are pushed by the explosion, the bullet will speed up much faster as it is lighter.
- The speed difference is by a factor of (weight shooter / weight bullet) and generally > 200.
- The explosion energy is put into both the shooter and the bullet. The energy is split unevenly and favors the fast bullet (by again the ratio of weight shooter / weight bullet). The bullet receives most energy, the shooter does not receive much energy in the recoil.
- Receiving the bullet in the bullet proof vest requires >200 times as much energy absorption compared to the recoil of shooting a bullet.
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