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Q & A: If you shoot a bullet straight up...

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Q:
Sir, If you shot a round bullet straight up (Vertically), is it true that when the bullet falls back to the same point where it was shot, it will have the same velocity(speed) as when it was fired?
- Ramon Gisbert (age 15)
Grace Cristian School, Clear Lake, Texas, USA
A:
Ramon -

The answer to your question is both yes and no.

In the non-existant world where only gravity matters, yes, the bullet would come back down at exactly the same speed as it went up. This is because gravity works to slow things down AND to speed things up. When the bullet is going up, gravity is trying to pull it down, so the bullet slows down. Eventually it slows down enough that it's not moving up any more. (This is when the bullet is at the top.) At this point, gravity is still pulling it down, so it starts to fall. Now, instead of slowing it down, gravity makes it go faster. Because gravity is constant (i.e. it doesn't change), the bullet will speed up while coming down at the SAME RATE as it slowed down when it was going up. So when it reaches the bottom, it will be going at the same speed as it was when it was first shot from the gun.

However, in the real world, gravity isn't the only thing effecting the bullet. In the real world, there's also air resistance. When gravity pulls on the bullet, it's always pulling it towards the ground, regardless of whether that is speeding up or slowing down the bullet. But when air restistance effects the bullet, it ALWAYS slows it down. This makes sense, as the air is basically just pulling against the bullet's motion. This means that in the real world, the bullet won't come down at the same speed that it went up, because air resistance will be slowing it down the whole way.

So, the answer is yes and no. With just gravity, yes, because the bullet speeds up on its way down the same amount as it slowed down on its way up. But air resistance slows it down the whole time, so in the real world, it will be slower when it comes back down than it was when it was first shot up.

-Tamara

(republished on 07/30/06)

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