Momentum against a wall

  • Thread starter kateman
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  • #1
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Homework Statement


Question: A stream of water from a hose is hitting a window. The window is vertical, the stream is horizontal, and the water stops when it hits. About 20cm(cubed), that is 20g, of water with speed 1.5 m/s strikes the window each second. Find a) the impulse on the window in time t and b) the force exerted on the window.


Homework Equations


I= delta p = mv -mu = Ft
f= delta p/t = mv-mu/t
s=ut +1/2 at (the last t is squared)

The Attempt at a Solution


well i originally tried to split this problem up into it vertical and horizontal components, saying that the water would follow a hyperbelar (i forget how to spell it) shape.

then i used the s=ut for the horizontal component (the rest of the equation isn't needed as the water doesnt accelerate horizontally and therefore is a 0 value) but then found that s and t arnt known.

i have a feeling it has something to do with the 20g of water with a force of 1.5m/s every second but i dont know what to do!

please help :)
 

Answers and Replies

  • #2
Doc Al
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i have a feeling it has something to do with the 20g of water with a force of 1.5m/s every second but i dont know what to do!
Indeed.

What's the momentum of the water that hits the window every second? What's its change in momentum, thus impulse. If that's for 1 second, what would it be for t seconds?
 
  • #3
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delta p = mv-mu
-20 x 1.5 = -30

okay thats for 1 second,
but its the t seconds thing that gets me

and thanks for helping
 
  • #4
Doc Al
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delta p = mv-mu
-20 x 1.5 = -30
Good.
okay thats for 1 second,
but its the t seconds thing that gets me
Think about it. If the impulse is 30 kg-m/s for 1 sec, what would it be for 2 seconds? 3 seconds? t seconds? (It's easy.)
 
  • #5
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so 30t

haha right, i cant believe i didn't get that before :P

one problem; the answer in the back of the book is 0.03t N s
 
  • #6
Dick
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That's because Doc and you were talking about 30kg of water and the problem says 30g.
 
  • #7
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lol thanks dick
 
  • #8
Doc Al
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That's because Doc and you were talking about 30kg of water and the problem says 30g.
D'oh! :redface: Thanks, Dick.
 

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