Paul Blay wrote:
> "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. ;-)
;)

i think now i can better explain myself (it's not a good idea to write a 
message to a newsgroup at 0:30AM ;)..

my problem is that he's mixing the effects of gravity and air resistance
(the 'the suck of the gravity...' quote)

and for the slowdown (for the bullet not reaching 3000km/h while 
falling), the only responsible element is the air resistance (and not 
the explosive-gases-are-stronger-than-gravity).

but i agree, that while it's poorly phrased, the conclusion can be 
correct (after all, 'false implies true' is true (if i remember it well :))

p.s: unfortunately, i have no idea about the dependency between the 
muzzle-length and bullet velocity ;(

gabor