Average speed

When an object moves in a straight line at a steady speed, you can calculate its average speed if you know how far it travels and how long it takes. The following equation shows the relationship between average speed, distance moved and time taken:

average~speed = \frac{distance~moved}{time~taken}

where:

average speed is measured in metres per second, m/s

distance moved is measured in metres, m

time taken is measured in seconds, s

For example, a car travels 300 m in 20 s. Its average speed is:

300 ÷ 20 = 15 m/s

Question

A man runs after a bus. The bus is travelling at an average speed of 5 m/s. The man runs 25 m in 6 s. Does he catch the bus?

The man’s average speed is 25 ÷ 6 = 4.2 m/s. So he will not catch a bus moving at 5 m/s.

To calculate the motion of everyday objects such as toy cars or tennis balls:

  1. measure the distance that the object travels in metres
  2. measure the time it takes for the object to travel that distance
  3. use the equation given above to calculate the average speed of the object