# Homework Help: Hydrostatic force on water tank problem

1. Feb 4, 2007

### Gauss177

1. The problem statement, all variables and given/known data
The end of a tank containing water is vertical and has the indicated shape (in attached picture). Explain how to approximate the hydrostatic force against the end of the tank by a Riemann sum. Then express the force as an integral and evaluate it.

2. Relevant equations
P=1000gx
F=P*A (pressure * area)

3. The attempt at a solution
I think my main problem is finding the area of the ith strip. The pressure is relatively easy to calculate, and if I know the pressure and area I can integrate to find the hydrostatic force.

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2. Feb 4, 2007

### AlephZero

Does the fact that the picture says "10 m (radius)" mean the end of the tank is a semicircle?

If so, you can use the equation of the circle, x^2 + y^2 = r^2, to find the area of each strip.

If it is some other shape, the question should tell you what the shape is.

3. Feb 4, 2007

### Gauss177

Yes, the end of the tank is a semicircle. Looking at a similar example in the book, is the length of each strip $$2 \sqrt{100-y_i^2}$$? Because in the book's example, the end of the tank is a full circle with radius 3, and they got the length of each strip to be $$2 \sqrt{9-y_i^2}$$. I'm not sure how to get this though from the equation of the circle.

Thanks

4. Feb 4, 2007

### AlephZero

The equation of the circle is $x^2 + y^2 = 10^2$

So $x = \pm\sqrt{100-y^2}$

The strip at height $y_i$ goes from $x = -\sqrt{100-y_i^2}$ to $x = +\sqrt{100-y_i^2}$ so its length is $2\sqrt{100-y_i^2}$

5. Feb 4, 2007

### Gauss177

Thanks. Can you quickly check if I did this right:

$$\displaystyle{\int_{0}^{5} 62.5y(2 \sqrt{100-y}) \;dy}$$
$$\displaystyle{125 \int_{0}^{5} y \sqrt{100-y} \;dy}$$

Substituting u=100-y, du=-dy:

$$\displaystyle{-125 \int_{100}^{95} (u+100) \sqrt{u} \;du}$$

I get the answer to be 1218880 lb for hydrostatic force. This seems really big, so did I do something wrong above?

Last edited: Feb 4, 2007