Page 1 of The hypothetical science question thread...

If a gun were to be fired straight up into the air, would the bullet come down with the same velocity/force that it was fired at (forgetting air resistance, etc.)?

If you forget air resistance etc then yeah - simple physics, it would have to come back down with the same total energy it went up with. Unfortunately you can`t forget air resistance. This one was covered by Mythbusters a little while back, see here:

http://kwc.org/mythbusters/2006/04/episode_50_bullets_fired_up_vo_1.html

The concluded that the bullet coming back down has nothing like the same energy as on the way up, but there are documented cases of people being killed by falling bullets.

I`d have thought that the bullet would only come down at a max of 120mph so the extra energy would have to be in the form of inertia (seeing as how the bullet would leave the gun at maybe 1000mph). Haven`t read the link yet btw. Cheers.

The max speed downward would be terminal velocity - ie it stops falling faster and travels at a constant rate (as with any falling object) - im guessing not likely to be the same as the speed of the bullet leaving the chamber of the gun

Quote:
simple physics, it would have to come back down with the same total energy it went up with.

not sure on your comment there. . . . . .
If you ignored air resistance would the bullet not slow down due to the pull of gravity and hence when it began to fall it would never recover the same momentum on the way down as it had leaving the gun as it doesnt have the "help" of the explosion from the gun


Can we ignore air resistance and gravity and just leave the bullet travelling straight up?

Chris

A.C.C. Member
LFC
YNWA

This item was edited on Friday, 3rd November 2006, 13:44

As already said (and in the link), terminal velocity is nothing like the starting velocity..

Ste

Quote:
max speed downward would be terminal velocity - ie it stops falling faster and travels at a constant rate (as with any falling object)

Not true actually. If the bullet could fall through the earth it would keep accelerating till it got to the centre of the earth where it would `float`. Gravity is equal to constant acceleration (and indistinguishable from it - ask Einstein ;) )

Still haven`t read the link yet - no time!!!

Not how I remember it from my physics degree


" Terminal velocity depends a great deal upon the shape of the object that is facing the direction it is moving. Once an object has reached terminal velocity, the object is not accelerating (a=0), therefore it is not speeding up or slowing down. It is a constant velocity unless the driving forces or the resistive forces change"


Chris

A.C.C. Member
LFC
YNWA

This item was edited on Friday, 3rd November 2006, 13:56

Quote:
If the bullet could fall through the earth it would keep accelerating till it got to the centre of the earth where it would `float`.For one, it can`t fall through the earth and two, even if it could it wouldn`t continue to accelerate. It would still reach terminal velocity. Upon reaching the centre it would float du to gravity from the opposite direction.
Don`t see why you think it would be different `inside` the earth - it`s already stopped accelerating before it reaches it. (9.81 m/s squared I seem to recall..)
Quote:
Gravity is equal to constant acceleration (and indistinguishable from it - ask Einstein ;) )Yes, in theory. But in practice, things don`t continue to accelerate..

Ste

i`m with sj (and 9.82m per second squared is indeed correct sir - dont doubt yourself)



Chris

A.C.C. Member
LFC
YNWA

Yeah, I probably got that stuff wrong. Mass of the earth constant an` all that.

"things don`t continue to accelerate"

they would if it were a black hole (up to speed of light any road)