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Dougan wrote:From t'internet:
When such a gun is fired straight up into the air, this is what happens:
The bullet leaves the gun at 3000 feet per second. There are two things now that slow the bullet down as it goes up. One is gravity - which slows the bullet 32 feet per second every second. If this were all that slowed it down, then after 10 seconds, the bullet would still be going (3000 - 320), or 2680 feet per second. This is not even close to the true velocity after 10 seconds, because air friction is slowing the bullet also. That friction is much greater while the bullet is going fast. In the first second, it may slow the bullet by 1000 feet per second. Then in the second second, maybe another 500, and then less and less, until finally, the gravitational slowing is more significant than the friction. The bullet will continue to slow down until it stops at the top.
Now let's talk about when the bullet heads back down:
The same two forces are present, but now the gravity part is speeding up the bullet - friction is still trying to slow it down. Gravity will be speeding up the bullet 32 feet per second every second, but friction will be increasing more and more as the bullet speeds up, and this friction will subtract from that 32. There is a certain velocity when the friction is equal to the gravity in the opposite direction. At this point, the frictional acceleration is 32 feet per second every second upward (or -32), while the gravitational acceleration is 32 feet per second every second downward (+32). From now on, the bullet will have no acceleration at all (32 - 32 = 0). When this happens, the velocity of the bullet is called its "terminal velocity." The bullet will continue at the same velocity until it hits the ground or some object.
This terminal velocity is different depending mostly on two things. 1. The density of the thing that is falling. 2. The shape of the thing that is falling. Bullets are very dense (usually lead), and they have a shape to allow relatively low friction. The terminal velocity of a feather may be only 2 feet per second because of its shape and density, but the terminal velocity of a bullet could be 300 or 400 feet per second. This could be about 300 miles per hour