Solution: Hydraulic engineers in the United States often use, as a unit of volume of water, the acre-foot, defined as the volume of water that will cover 1 acre of land to a depth of 1 ft. A severe thunderstorm

Solution: Hydraulic engineers in the United States often use, as a unit of volume of water, the acre-foot, defined as the volume of water that will cover 1 acre of land to a depth of 1 ft. A severe thunderstorm

Hydraulic engineers in the United States often use, as a unit of volume of water, the acre-foot, defined as the volume of water that will cover 1 acre of land to a depth of 1 ft. A severe thunderstorm dumped 2.0 in. of rain in 30 min on a town of area 26 km^{2}. What volume of water, in acre-feet, fell on the town?

Whenever we convert units, the first step is to figure out what our starting and ending units are. We'll place the *starting units on the left*, an equals sign and *ending units on the right*, and some *conversion factors in between*.

Also, remember that if one of the starting units has an *exponent* (like **m**^{3} or **s**^{2}), the conversion factors for that unit *also* need to have the same exponent. For example:

$\left(\frac{{s}{t}{a}{r}{t}{i}{n}{g}{}{u}{n}{i}{t}}{{s}{t}{a}{r}{t}{i}{n}{g}{}{u}{n}{i}{{t}}^{{2}}}\right)\times \left(conversionfactor\right)\times {\left({c}{o}{n}{v}{e}{r}{s}{i}{o}{n}{}{f}{a}{c}{t}{o}{r}\right)}^{2}=\left(\frac{{e}{n}{d}{i}{n}{g}{}{u}{n}{i}{t}}{{e}{n}{d}{i}{n}{g}{}{u}{n}{i}{{t}}^{{2}}}\right)$

The conversion factors *must* *cancel out* the starting unit and *leave* the ending unit. So to cancel out the starting unit in the **numerator**, the first conversion factor must have that same unit in the **denominator**.

The exact opposite happens with the starting unit in the **denominator**. To cancel it, the second conversion factor must have that same unit in the **numerator**. *Other conversion factors may be necessary*. Once all the units have canceled and you’re left with the ending units (both numerator and denominator), multiply and divide all the numbers through.

We want to express the volume in acre-feet (acre•ft), but we're given square kilometers and inches. So what we'll do here is convert km^{2} to acres and inches to, then multiply those results together to get the final answer. (You can also do the whole conversion on one line.)

*(You might notice that the problem also gives us a time measurement of 30 minutes, but we don't need that information to answer the question. Real-life data often works that way, and some physics problems try to do the same thing so students get used to the idea.)*