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Wolf of the Red Moon's question at Yahoo! Answers regarding the computation of work

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Feb 24, 2012
Here is the question:

Need help with a Calculus Work problem...have answer not sure how to get to it?

A cylindrical gasoline tank 3ft in diameter and 4ft long is carried on the back of a truck and used to fuel tractors in the field. The axis of the tank is horizontal. Find the work done to pump the entire contents of the tank into a tractor if the opening on the tractor's tank is 5ft above the top of the tank in the truck. Assume gasoline weighs 42 lbs per cubic foot.

(Hint: Evaluate one integral by a geometric formula and the other by observing that the integrand is an odd function.)
I have posted a link there to this topic so the OP can see my work.
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Feb 24, 2012
Re: Wolf of the Red Moon's question regarding the computation of work

Hello Wolf of the Red Moon,

I prefer to work problems like this in general terms, and then plug our given data into the resulting formula.

First, let's let:

$r$ = the radius of the tank

$\ell$ = the length of the tank

$q$ = the distance above the top of the tank the fluid must be pumped

Now, let's imagine slicing the contents of the tank horizontally into rectangular sheets. The length of each sheet is constant, given by the length of the tank $\ell$. The width $w$ of each sheet will be a function of its vertical position within the tank.

So, let's orient a vertical axis, with its origin at the bottom of the tank, as in the diagram:


We wish to find $w$ as a function of $y$. We should observe that we may state:

\(\displaystyle (y-r)^2+\left(\frac{w}{2} \right)^2=r^2\)


\(\displaystyle w(y)=2\sqrt{r^2-(y-r)^2}\)

and so the volume of an arbitrary sheet is:

\(\displaystyle dV=\ell\cdot w(y)\,dy=2\ell\sqrt{r^2-(y-r)^2}\,dy\)

Next, we want to determine the weight $\omega$ of the arbitrary sheet. So let's define $\rho$ to be the weight density of the fluid, where:

\(\displaystyle \rho=\frac{\omega}{dV}\,\therefore\,\omega=\rho\,dV\)


\(\displaystyle \omega=2\rho\ell\sqrt{r^2-(y-r)^2}\,dy\)

Next, we want to determine the distance $d$ the arbitrary sheet must be lifted. This is:

\(\displaystyle d=q+(2r-y)\)

Thus, using the fact that work is the product of the applied force and the distance through which this force is applied, we find the work done to lift the arbitrary sheet is:

\(\displaystyle dW=\omega d=(q+(2r-y))\left(2\rho\ell\sqrt{r^2-(y-r)^2}\,dy \right)\)

In order to utilize the given useful hint, we may arrange this as:

\(\displaystyle dW=\omega d=((q+r)-(y-r))\left(2\rho\ell\sqrt{r^2-(y-r)^2}\,dy \right)\)

Now, summing by integration from the bottom of the tank $y=0$ to the top $y=2r$, we may state:

\(\displaystyle W=2(q+r)\rho\ell\int_0^{2r}\sqrt{r^2-(y-r)^2}\,dy-2\rho\ell\int_0^{2r}(y-r)\sqrt{r^2-(y-r)^2}\,dy\)

Now, by geometry, we should observe that:

\(\displaystyle 2\int_0^{2r}\sqrt{r^2-(y-r)^2}\,dy=\pi r^2\)

Notice this is simply the area of a circle of radius $r$.

And we may also observe that the integral:

\(\displaystyle \int_0^{2r}(y-r)\sqrt{r^2-(y-r)^2}\,dy\)

may be rewritten using the substitution:

\(\displaystyle u=y-r\,\therefore\,du=dy\)

and we have:

\(\displaystyle \int_{-r}^{r} u\sqrt{r^2-u^2}\,du\)

Since the integrand is an odd function, and the interval of integration symmetric about the origin, we may state by the odd function rule:

\(\displaystyle \int_{-r}^{r} u\sqrt{r^2-u^2}\,du=0\)

Putting this all together, we have:

\(\displaystyle W=\pi(q+r)r^2\rho\ell\)

Now, plugging in the given data:

\(\displaystyle q=5\text{ ft}\)

\(\displaystyle r=\frac{3}{2}\text{ ft}\)

\(\displaystyle \rho=42\,\frac{\text{lb}}{\text{ft}^3}\)

\(\displaystyle \ell=4\text{ ft}\)

we find:

\(\displaystyle W=\pi\left(\left(5+\frac{3}{2} \right)\text{ ft} \right)\left(\frac{3}{2}\text{ ft} \right)^2\left(42\,\frac{\text{lb}}{\text{ft}^3} \right)\left(4\text{ ft} \right)=2457\pi\text{ ft}\cdot\text{lb}\)