## Thursday, May 24, 2012

### Feet Cubed....

As every high school student OBVIOUSLY knows, "3 feet cubed" is "3x12 inches cubed". Duh! You know, you just multiply by 12 because there are 12 inches in a foot.

Like everyone else, I've tried various ways to dispel this rumor: we've drawn models, we've used models, we've discussed. Seemingly to no avail. This year, I tried yet again with a different tactic.

We started out with a rectangular prism 3' x 2' x 4'. They found the volume in feet cubed. Then I asked them to find the volume in inches cubed, and to a person, they all wrote the wrong answer (24 x 12 = 288). Then without saying they were wrong I had them go back and redraw the prism into the inches equivalent and recalculate the volume, and OH NO, they were wrong with their first "288". Hmmmmm. So, this is stuff I've tried before. Now for the "new" attempt that MAYBE will work.

I had them write: 5 ft^3
Then change to: 5 * ft * ft * ft
Then EACH foot had to be converted to inches, so

= 5 * ft * ft * ft
= 5 * 12in * 12in * 12in
= 8640 in*in*in
= 8640 in^3

Let's see if this one sticks.

1. Good idea--let us know how it works!

2. The linguist in me (yeah, I double majored in math and linguistics) discourages me from saying "feet cubed" in favor of "cubic feet" since in English the adjective comes before the noun...

...but I will say that what you are doing is maybe the most easily understood way (that I've seen) of converting square or cubic feet.

I bet it'll work well.