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        riddles >> putnam exam (pure math) >> rate of momentum discharge
        
(Message started by: Larissa Preedy on Jun 7th, 2005, 6:40am)
Title: rate of momentum discharge
Post by Larissa Preedy on Jun 7th, 2005, 6:40am
hey guys,

having some trouble with this question i have.. can someone give me some pointers on how to solve this? or the answer if u want ;)

A fireman holds a water nozzle. The diameter of the nozzle is 0.065999m and the water exit velocity is 6.2m/s. If the density of the water is 1016 kg/m^3, what is the rate of momentum discharge from the nozzle?
Title: Re: rate of momentum discharge
Post by towr on Jun 7th, 2005, 10:14am
1 s of water is 6.2 x  pi x (0.065999/2)^2 m3 ~= 0.02121 m3

0.02121 m3/s *  1016 kg/m3 ~= 21.55 kg/s

21.55 kg/s * 6.2 m/s ~= 133.6 (kg m/s)/s

Though frankly I'm just guessing at what 'rate of momentum discharge' means.
Title: Re: rate of momentum discharge
Post by Icarus on Jun 7th, 2005, 6:23pm

on 06/07/05 at 10:14:51, towr wrote:
Though frankly I'm just guessing at what 'rate of momentum discharge' means.


Yes - the framer of this question should be slapped a few times and told to speak proper technical English, not self-created jargon.
Title: Re: rate of momentum discharge
Post by Larrisa on Jun 7th, 2005, 8:45pm

on 06/07/05 at 10:14:51, towr wrote:
1 s of water is 6.2 x  pi x (0.065999/2)^2 m3 ~= 0.02121 m3

0.02121 m3/s *  1016 kg/m3 ~= 21.55 kg/s

21.55 kg/s * 6.2 m/s ~= 133.6 (kg m/s)/s

Though frankly I'm just guessing at what 'rate of momentum discharge' means.


omg, that's exactly what i did and i kept gettin it wrong :( i was getting something around 534 :S but i can't see what you did difference.

The units are kgm/s2 correct?

and thanks heapS!
Title: Re: rate of momentum discharge
Post by towr on Jun 8th, 2005, 2:40am
Yes, or in other words N, newton.
I put it explicitly as moment per second, as that seems to be what was asked for.
It should just be equal to the force with which the water is pushed out of the hose.

Oh, and as for what you did wrong. You seem to be off a factor 4. Which is neatly explained if you used the diameter of the nozzle instead of the radius (half the diameter) in the calculation.
pi r2 = 1/4 pi d2
Title: Re: rate of momentum discharge
Post by Larissa_Preedy on Jun 8th, 2005, 2:55am

on 06/08/05 at 02:40:33, towr wrote:
Oh, and as for what you did wrong. You seem to be off a factor 4. Which is neatly explained if you used the diameter of the nozzle instead of the radius (half the diameter) in the calculation.
pi r2 = 1/4 pi d2



omg, what a silly silly mistake!!!  :-[ :-[ :-[
Title: Re: rate of momentum discharge
Post by towr on Jun 8th, 2005, 4:04am
Meh, beats mistaking feet for meters (or vice versa) and crashing a satellite into mars.  ;D
Title: Re: rate of momentum discharge
Post by Larissa_Preedy on Jun 8th, 2005, 6:57am
LOL


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