How long does a bullet take to hit the ground?

02 November 2008

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Question

My question is about rifles and bullets. Some scientific friends of mine assured me that a bullet fired from a rifle held horizontally will hit the ground at exactly the same time as a bullet from a rifle held vertically, pointing downwards. Fair enough, I’m a linguist. I was rotten at science but that seems to defy common sense.

Answer

Dave Ansell answered question...

Dave - This is a mangling of quite a famous experiment. What the actual thing they should have told you is that if you fire a rifle horizontally and "drop" a similar bullet from the same height as the gun and at the same time as you fire the gun, then they'll both hit the ground at the same moment.

Chris - So you've got a bullet in one hand, rifle in the other. You fire the gun at the same moment you let go of another bullet from your hand. The two bullets should actually hit the ground...

Dave - At exactly the same time. If there's no air resistance, basically how fast you're going horizontally has no relationship to how fast you accelerate downwards. The bullet which is moving from the gun and the one which is dropped both accelerate downwards under the influence of gravity at exactly the same speed. So they'll both hit the ground at the same time. If you take a gun and fire it straight downwards then the bullet's going to come out of the gun at several hundred metres per second. That means it's going to hit the ground far quicker than the one you fire horizontally!

Chris - We did an amazing experiment at school, which I remember to this day, which shows how good this experiment was: the monkey and hunter experiment. We had a sort of blow-pipe with tin-foil across the end of the blow pipe; this was making an electrical contact to an electro-magnet that was holding up a tin can at a distance.

A ball bearing was put into the glass tube. You blow down the glass tube so the ball bearing leaves the blow pipe, breaking the piece of foil in the process. This cuts the supply to the electromagnet. The tin can then starts to fall at exactly the same rate as the ball bearing leaves the tube. If all things are equal, i.e. the can is being accelerated down by gravity at the same rate as the ball bearing is being accelerated down by gravity the two will hit each other. And they always do!

It's called the monkey and hunter experiment because the idea is that the monkey is dangling in the distance on a tree and the hunter fires the gun. Assuming it takes no time for the sound of the gun discharging to reach the monkey, the monkey lets go of the tree and starts to drop at the same moment that the bullet leaves the gun. Therefore the bullet's falling and the monkey's falling and they should still reach each other. It's a very elegant way of explaining it!

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