Monday, 11 February 2013

Escaping Bullets in Water

So you are in a place where there is water.  This could be on an island, on a dock, on a lake or possibly even a swimming pool.  Suddenly someone pools a gun.  You think about all the action movies like Lethal Weapon 3 where the hero dives into the water to escape being shot to death.

Can the surface tension of water help you to survive?

The water can probably help you depending on the speed of the bullet fired in the gun.  It might not have anything to do with the surface tension of the water although that may help slow the bullet down on first impact..  If you fire a bullet into water one of two things are sure to happen:

1. the bullet ricochets of the surface due to the surface tension of the water.
2. the bullet shatters on the force of impact trying to break the surface tension. even an aluminum bullet will shatter.

However, sometimes a situation may arise that the bullet does not shatter and will break the surface tension in one piece.  The bullet might get a little bit destroyed so ballistics test could not identify it.  It also might still penetrate your body if it is one piece.  However, the surface tension will redirect the bullet and make it more difficult for the person shooting to take an accurate shot.  That is how people who are being shot at and take refuge in water don’t getting fatally wounded.

How can water be that bullet proof?  Mythbusters does an alright albeit (layman) job at showing how the bullet proof water actually works. Here is a video.

What did I learn?  If I’m ever being shot at, I’m looking for the nearest body of water!
Shooting into water at an angle of 23 degrees you need to only be two feet under and three feet away to survive being shot at, even by something as big as a 0.50 caliber weapon.  Most of the bullets would disintegrate into a handful of shrapnel on water impact and fall harmlessly to the bottom.  Even with very powerful weapons like the M1 the 223 you only need to be in 3 ft of water in order to escape the bullet.  So the stronger the gun the less water you need.  Modern bullets move much faster that the water by comparison moves much more like a solid than a liquid, causing the bullet to self-destruct.  So the much slower civil war bullet gave the water in front of it enough time to move out of the way.  This allowed the bullet to go much farther.  

The Mythbusters, as in many of their shows, do not really explain what the forces are behind their experiment though. Although it might seem at first that this has something to do with surface tension.  It has less to do about surface tension than the overall surface pressure of the bulk water.  To test whether surface tension has an affect one solution would be to raise and lower the temperature of the water or to add a surfactant.  Likely this has little impact and the surface pressure might be the larger effect.  To test the latter the pool size could be changed.