Catching a bullet in your teeth
Question:Is it possible to catch a bullet in your teeth, and if not, is it provable with physics?
Answer:
I wouldn't like to try. I value my teeth too much! However lets look at the Physics behind it.
The impact of the bullet with your teeth involves two important conservation laws of Physics:
(a) energy
(b) momentum
Imagine a bullet of mass 100 grams (0.1 kg) travelling at 200 m/s. Quite likely - many will travel faster – i.e. greater than the speed of sound. They always say that you don't hear the bullet that hits you.
(a) Energy – the kinetic energy of my example bullet is ½ mv2 = ½ x 0.1 x 2002 = 2000 J
Now this energy has to go somewhere. The possibilities are:
(i) sound as it hits your teeth
(ii) to kinetic energy of your whole body as you are knocked backwards and
(iii) in breaking your teeth
(b) Momentum
The momentum before the collision = the momentum after collision.
Lets suppose you do catch the bullet and that your mass is 75 kg. (I just have to make a guess at that – it is my mass (165 lb).
The momentum of the bullet is transferred to the whole of your body (and a little extra for the mass of the bullet but this is negligible) and you move backwards but how fast?
0.1x200 = 75 x v where v is the velocity with which you move.
This gives v = 0.27 m/s or 27 cm/s. So not too fast but significant.
Now we come to the force on your teeth as the bullet is slowed from 200 m/s to 0.27 m/s.
Force = momentum change/time taken. Imagine it takes you 0.15 s to slow the bullet down. This will give a force of (200x0.1 – 0.27x0.1)/0.15 = 133 N. Not massive and something your teeth could stand.
However now to the real problem.
How will you know just when to bite on the bullet as it flies between your teeth at 200 m/s (that is over 400 mph)? It will need amazingly fast reactions!
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