Question: Impulse and Force on a Vertical Window Struck by a Stream of Water

[kateman]

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 doesn't 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 don't know what to do!

please help :)

 

[Doc Al]

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 don't 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?

 

[kateman]

delta p = mv-mu
-20 x 1.5 = -30

okay that's for 1 second,
but its the t seconds thing that gets me

and thanks for helping

 

[Doc Al]

delta p = mv-mu
-20 x 1.5 = -30

Good.

okay that's 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.)

 

[kateman]

so 30t

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

one problem; the answer in the back of the book is 0.03t N s

 

[Dick]

That's because Doc and you were talking about 30kg of water and the problem says 30g.

 

[kateman]

lol thanks dick

 

[Doc Al]

That's because Doc and you were talking about 30kg of water and the problem says 30g.

D'oh! Thanks, Dick.