2.5 Average Speed

A description of how fast or slow an object moves is its speed. Speed is the rate at which an object changes its location. The SI unit of time is the second (s), and the SI unit of speed is metres per second (m/s), but sometimes kilometres per hour (km/h), miles per hour (mph) or other units of speed are used.

When you describe an object’s speed, you often describe the average over a time period. Average speed, [latex]V_{avg}[/latex], is the distance traveled divided by the time during which the motion occurs.

[latex]\begin{align*}V_{avg}=\frac{\text{distance}}{\text{time}}\end{align*}[/latex]

You can, of course, rearrange the equation to solve for either distance or time:

[latex]\begin{align*}\text{time}&=\frac{\text{distance}}{V_{avg}}\\[1ex]\text{distance}&=V_{avg}\times\text{time}\end{align*}[/latex]

Suppose, for example, a car travels 150 kilometres in 3.2 hours. Its average speed for the trip is

[latex]\begin{align*}V_{avg}&=\frac{\text{distance}}{\text{time}}\\[1ex]&=\frac{150\;\text{km}}{3.2\;\text{h}}\\[1ex]&=47\;\text{km/h}\end{align*}[/latex]

A car’s speed would likely increase and decrease many times over a 3.2 hour trip. Its speed at a specific instant in time, however, is its instantaneous speed. A car’s speedometer describes its instantaneous speed.

Attribution

“2.2 Speed and Velocity” from Physics by Paul Peter Urone, Roger Hinrichs, © OpenStax is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.