Convert Miles per Hour to Feet per Second (Unit Cancellation)
Convert 2 miles per hour to feet per second:
2 miles X 5,280 feet X 1 hour X 1 minute = 2.93 feet per second
1 hour 1 mile 60 minutes 60 seconds
We want to show you a method for doing calculations that won’t require you to memorize a lot of
formulas. It’s called Unit Cancellation, with units being the units of measurement, like inches, feet,
seconds, ounces, you get the picture.
We can use unit cancellation to solve many problems or answer many questions, by using the
relationship of one unit to another. For example, 12 inches = 1 foot. When you are working through a
problem, always write the units. This will help you set up the units as fractions or ratios, and tells you
whether to multiply or divide. Any whole number can be turned into a fraction by putting 1 under it
Convert 15 yards to inches, knowing that 1 yard = 3 feet, and 1 foot = 12 inches.
15 yards X 3 feet X 12 inches = 540 inches
1 1 yard 1 foot
Example: Convert 2 miles to inches. We want our final answer in inches. We can use what we know to
develop a path… we know how many feet in a mile, and how many inches in a foot. We start out by
writing:
2 miles Any whole number can be turned into a fraction by putting 1 under it.
1
Then, we want to convert miles to feet, knowing that 1 mile = 5,280 feet. Here is where the trick starts.
We want the units to cancel out, so set up the next ratio, or fraction, with miles on the bottom, so it will
cancel out.
2 miles X 5,280 feet
1 1 mile
2 miles X 5,280 feet = 10,560 feet
1 1 mile
The next step is to convert feet to inches. We know that 1 foot = 12 inches. Set up the next ratio
(fraction) so that feet are on the bottom and inches on the top, so they again cancel out:
10,560 feet X 12 inches = 126,720 inches
1 1 foot
