Re: Does this count as irony?
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
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