Science Questions

Sun, 15th Jan 2006

Part of the show Plant Science, Composting and Mosquito Repellents

Question

Eddie in Essex asked:

It's always intrigued me that someone can have an accident, crash into a fence that pierces their body, and survive without any problems, and yet a little bullet can pierce the body and kill you. Why is that?

Answer

It's all down to the momentum of the thing concerned. The Stardust mission comet Kat spoke about earlier was actually travelling thousands and thousands of miles and hour faster than the probe was, and the tiny particles it was throwing off were capable of destroying that probe. And this is all down to momentum. The momentum of something is the mass of something multiplied by how fast it's going. That's how hard something slams into you. So it doesn't actually matter how heavy or big something is because the faster it hits you, the harder it hits you. If you drive into a fence at 60 miles per hour, the slamming effect will be the weight of your body multiplied by the speed that you're going. If you're hit by a bullet, although the mass of the bullet is much less than you hitting the fence post, it's going a lot faster. The average bullet coming out of a pistol is going at around 700 miles per hour, which is a lot faster than someone in a racing car. As a result, the momentum of a bullet is much higher. The reason bullets cause such a problem is because although they go in and make a very small entry wound, the shock wave of the bullet hitting your tissues rips you apart internally. That's why if you look at someone who's been shot from the front, they don't look too bad until you look at them from the back where there's often a very big exit wound. This is because as the bullet goes through, it compresses all the tissues it passes through and pulls them to pieces. It's like being hit with a fence post but much much harder.

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