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Name: David
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Question:
In the real world,(not the physics world of no air resistance), how do I calculate the speed of a bullet that was shot straight into the air when it returns to Earth. For example, say a rifle had a muzzle velocity of an M-16, what would the bullet's speed when it comes straight back down. On the news one sees celebrations in the Middle East of shooting into the air. Just how dangerous is that?



Replies:
When falling, every object has a "terminal speed" when falling. If this were not true, a falling raindrop would be moving fast enough to kill you. It works exactly the same as a parachute, exactly the same. The only difference is that the parachute is much wider. A parachute makes things fall slower because it is much wider. The parachute must push much more air out of the way as it falls. To know the terminal speed requires much more information than the fact that it is a bullet. Other factors are: mass of the bullet, cross-sectional area of the bullet, humidity and density of the air. Still we can say something. I expect a bullet is heavier than a raindrop, thus increasing the terminal speed. I also expect that a bullet is wider than a raindrop, thus slowing it down. Therefore, as a first estimate, the bullet is probably on the same order of magnitude as a raindrop. A raindrop falls at about 6 m/s. A bullet would thus fall on the order of 10 m/s. I would expect it could be as high as 50 m/s.

Dr. Mellendorf



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