# Momentum against a wall

## 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

Doc Al
Mentor
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?

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

Doc Al
Mentor
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.)

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

Dick
Science Advisor
Homework Helper
That's because Doc and you were talking about 30kg of water and the problem says 30g.

lol thanks dick

Doc Al
Mentor
That's because Doc and you were talking about 30kg of water and the problem says 30g.
D'oh! Thanks, Dick.