"Paul Blay" <ask_me_or_get_spam_trapped@saotome.demon.co.uk> wrote in 
message news:ddtgvl$1t5$1$830fa7b3@news.demon.co.uk...
> "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. ;-)

The muzzle velocity isn't important, just the velocity of the bullet on it's 
way back down.