# Homework Help: Conversion and volume

1. Jun 23, 2005

### Enos

The question is about converting 26km^2 with 2 inches of water into acre-feet. I know the answer is 1100 acre-feet but stuck in converting feet into acre-feet.

1 acre = 43560ft^2
1 acre-foot = 1 acre of land and one foot depth.

I might be posting here often and just wondering if it's ok. This isn't homework for college or school and I bought a text book by haliday (7th edition) so I can learn physics at home.

Thanks for any help.

2. Jun 23, 2005

### OlderDan

An acre is a unit of area. An acre-foot is a unit of volume. Start by writing the volume of the water in the units given. Once you have that, unit conversion is accomplished by multiplying by factors equal to one to replace unwanted units in favor of desired units. For example, you might want to multiply your original quantity by one of the two factors:

$$\frac{1 ft}{12 in}$$

or

$$\frac{12 in}{1 ft}$$

Both factors are equal to 1, so multiplication will not change the value of the quantity, but will change the units in which that quantity is expressed. Any factor equal to one can be used in this manner. Unit conversion often involves multiplying by several such factors.

3. Jun 27, 2005

### Enos

I think I got the right answer. I just disagreed with the text book because I forgot to round the number to two significant figures. the answer I got was 1070 and the answer in the text book was 1.1x10^3
All I did was 26x1000^2x3.281^2x(1/12)/43560

4. Jun 27, 2005

### OlderDan

Actually, what you did was

$$V = 26 km^2 \cdot 2 in \cdot \left[ \frac{1000\ \ m}{km} \right]^2\cdot \left[ \frac{3.281\ \ ft}{m} \right]^2 \cdot \left[ \frac{1\ \ft}{12\ \ in} \right] \cdot \left[ \frac{1\ \ acre}{43560\ \ ft^2} \right] = 1071\ \ acre-feet$$

Besides leaving out all the units, you left out the 2 in your posted equation. Writing the units is always a good idea and will help you keep track of what you are doing.

5. Jun 28, 2005

### Enos

Whoops, meant to put (2/12) in my posted equation rather than (1/12). I'm starting to get a hang of this stuff.