Calculating Water Speed in a River Rapids

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Hello, I would appreciate any help!

Homework Statement


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.


Homework Equations


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

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?
 
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dislike said:
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.

Hi dislike! Welcome to PF! :smile:

You don't need the pressure.

Just use conservation of mass. :wink:
 
I'm still confused; could you explain a little more.
 
l understand what you mean now. Thank You.