Unit Conversions: A Short Drive Home (part 1)

First we calculate the average speed using the given units. Then we can get the average speed into the desired units by picking the correct conversion factor and multiplying by it. The correct conversion factor is the one that cancels the unwanted unit and leaves the desired unit in its place.

1. Calculate average speed. Average speed is distance traveled divided by time of travel. (Take this definition as a given for now—average speed and other motion concepts will be covered in a later module.) In equation form,

   $$\text{average speed = }\frac{\text{distance}}{\text{time}}\text{.}$$

2. Substitute the given values for distance and time.

   $$\text{average speed = }\frac{10.0\text{ km}}{20.0\text{ min}}=0.500\frac{\text{km}}{\text{min}}$$

3. Convert km/min to km/h: multiply by the conversion factor that will cancel minutes and leave hours. That conversion factor is $60\text{min/1}\text{h}$ . Thus,

   $$\text{average speed = }0.500\frac{\text{km}}{\text{min}}\times \frac{60\text{ min}}{1\text{ h}}=30.0\frac{\text{km}}{\text{h}}.$$