Calculating Water Speed in a River Rapids

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To calculate the speed of water in the river rapids, apply the principle of conservation of mass rather than focusing on pressure. Given the river's narrowing from 12m to 5.8m and the change in depth from 2.7m to 0.85m, the flow rate must remain constant. The initial speed of 1.2m/s can be used to determine the speed in the narrower section by equating the flow rates before and after the rapids. The discussion emphasizes that pressure calculations are unnecessary for this problem. Understanding these principles will lead to the correct calculation of water speed in the 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.
 

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