# Homework Help: Speed of Water Flowing

1. Oct 7, 2008

### dislike

Hello, I would appreciate any help!
1. The problem statement, all variables and given/known data
A river narrows at a rapids from a width of 12m to a width of only 5.8m. The depth of the river before the rapids is 2.7m; the depth in the rapids is .85m. Find the speed of water flowing in the rapids, given that its speed before the rapids is 1.2m/s. Assume the river has a rectangular cross section.

2. Relevant equations
I'm using bernoulli's equation:
P1+pv^2/2+pgy=P2+pv2^2+pgy2
and P=F/A

3. The attempt at a solution
I figured I would have to find the Pressures at the top and the bottom with P=F/A. The area I can find with the depth and width given but how do I find the force. Or is the pressure on the top of the river just atmospheric pressure?

2. Oct 7, 2008

### tiny-tim

Welcome to PF!

Hi dislike! Welcome to PF!

You don't need the pressure.

Just use conservation of mass.

3. Oct 7, 2008

### dislike

I'm still confused; could you explain a little more.

4. Oct 7, 2008

### dislike

l understand what you mean now. Thank You.