# Homework Help: Dam problem

1. Jan 22, 2004

### rdn98

Picture is included.

You are working as a summer intern for the Illinois Department of Natural Resources (DNR) and are assigned to some initial work on water resources project. The department will be overseeing the construction of a dam to create a large fresh water lake that will be approximately 18 meters deep. A horizontal pipe 1.2 meters long and 5 cm in diameter will pass through the dam at a depth of 6 meters to allow for release of the water in emergencies and for sampling. In normal situations, a plug will secure the pipe opening.
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(a) What will be the hydrostatic force on the plug?

How do I setup the equation for a hydrostaic force? I've tried various FBD, but none is getting me anywhere.

Ok, I know that at the same horizontal level of the pipe, the pressure is constant at that depth.

FBD of the plug shows a mg down, a bouyant force pointing to the right, and normal force up from the dam, and a foce pushing the plug down from the dam also.

So the forces in y direction cancel out. And I assume that the hydrostatic force would be the same thing as the bouyant force for this question? I am stuck and need help badly.

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2. Jan 22, 2004

### Staff: Mentor

I think you are making too much out of a simple problem. The hydrostatic force on the plug is just the force due to the water pressure. Find the pressure at depth D2 and then calculate the force. (Ignore atmospheric pressure--since it's the same on both sides of the plug.)