"Gabor Farkas" <gabor@nekomancer.net> wrote ...
> Simon wrote:
>> http://www.abc.net.au/science/k2/homework/s95523.htm 
>
> sorry, but this is...hmm...i wonder if he really does not know it, or he 
> is trying to explain it using simpler terms and because of that he says 
> something wrong....

> and since when does the weight of an item affect the speed of it's fall?

"depending upon the weight and shape of the bullet."

Actually his comment is fair as long as you include _size_ as a component 
of shape.  That's a bit dubious but it also works if the bullets are of 
about the same density.

> yes, the explosive gases maybe represent a greater force, but they stop 
> affecting the bullet after it left the gun. on the other hand, the 
> gravity affects the bullet the whole time.

"The suck of gravity is not as powerful as the explosive gases that push 
it out of the barrel. So it will accelerate to a maximum speed of not 
3,000 kilometres per hour, but somewhere between 330 and 770 kilometres 
per hour"

Again this is poorly phrased but if you look at it this way ...
The "explosive gases", against any resistantive forces (e.g. side of gun barrel) 
accelerate it to 3,000 km per hour by the time it reaches the end of the barrel.

The maximum speed from /gravity/ will depend on the gravitational force 
equalling the resistive force from air.  This is where the "330 to 770 km / hour"
bit comes from.  

Now the trick is (which I don't know the answer to) how does bullet 
velocity vary with muzzle length?  The longer the muzzle the greater 
time the gas can act on the bullet to accelerate it but also the force 
from the gas will decrease the further the bullet is along the muzzle.

If the bullet has ceased, or almost ceased, accelerating by the time it
reaches the end of the muzzle then his statement is a lot more accurate.

So although you're right about the relative time that the forces apply to the 
bullet the final velocity (down) doesn't depend on the muzzle velocity but 
(given a few assumptions, including that it is fired directly up) will depend
solely on the bullet density, weight and shape.  Also the muzzle velocity
(up) /could/ be that where the gas force is equalized by the resistance 
forces.  In which case he might validly compare 'force with force' instead 
of 'impulse with impulse'

If I was going to bet though I'd bet that the bullet hasn't reached constant 
velocity at discharge from the muzzle - No doubt some U.S. poster will 
know all the ins and outs of it. ;-)