Students will learn how to convert units from metric to English system and vice versa using dimensional analysis.
- The key to converting units is to multiply by a clever factor of one. You can always multiply by 1, because it does not change the number. Since 1 in. is equal to 2.54 cm, then . Thus, one can multiply by this form of 1 in order to cancel units (see video below).
- Write out every step and show all your units cancelling as you go.
- When converting speeds from metric to American units, remember the following rule of thumb: a speed measured in mi/hr is about double the value measured in m/s ( i.e., is equal to about 20 MPH). Remember that the speed itself hasn’t changed, just our representation of the speed in a certain set of units.
- When you’re not sure how to approach a problem, you can often get insight by considering how to obtain the units of the desired result by combining the units of the given variables. For instance, if you are given a distance (in meters) and a time (in hours), the only way to obtain units of speed (meters/hour) is to divide the distance by the time. This is a simple example of a method called dimensional analysis , which can be used to find equations that govern various physical situations without any knowledge of the phenomena themselves.
Question : 20 m/s = ? mi/hr
20 m/s (1 mi/1600 m) = .0125 mi/s
.0125 mi/s (60 s/min) = .75 mi/min
.75 mi/min (60 min/hr) = 45 mi/hr
Watch this Explanation
Time for Practice
Estimate or measure your height.
- Convert your height from feet and inches to meters.
- Convert your height from feet and inches to centimeters
Estimate or measure the amount of time that passes between breaths when you are sitting at rest.
- Convert the time from seconds into hours
- Convert the time from seconds into milliseconds
- Convert the French speed limit of into .
Estimate or measure your weight.
- Convert your weight in pounds into a mass in
- Convert your mass from into
- Convert your weight into Newtons
- Find the unit for pressure.
An English lord says he weighs
- Convert his weight into pounds (you may have to do some research online)
- Convert his weight in stones into a mass in kilograms
- If the speed of your car increases by every 2 seconds, how many is the speed increasing every second? State your answer with the units .
- a. A person of height . . is tall b. The same person is
- a. b.
- If the person weighs then a. 67.9 kg (on Earth) b. 67.9 billion c. this is equivalent to
- Pascals (Pa), which equals
- a. .,b.