Speed, distance and time

The speed of an object tells you how fast or slow it is moving. You can find the average speed of an object if you know:

  • the distance travelled
  • the time taken to travel that distance

You can calculate average speed using this equation:

curriculum-key-fact
average speed = distance ÷ time

Example 1

Question

Calculate the average speed of a runner who runs 100 m in 10 s.

average speed = 100 ÷ 10 = 10 m/s

Notice that the unit for speed in science is metres per second, m/s. It is not, for example, mph, kph or m per s.

If you are given the distance travelled in km, multiply it by 1000 to get the distance in m. For example, 3.5 km is 3500 m (3.5 × 1000).

Example 2

Question

A car travels 2 km in 100 s. Calculate its average speed.

2 km = 2 × 1000 = 2000 m

average speed = 2000 ÷ 100 = 20 m/s

Average speed cameras

Speed cameras are used to find out if a motorist is travelling faster than the speed limit for the road. The camera takes two photographs of the vehicle. These can be:

  • a certain time apart, so that the distance travelled in that time can be worked out, or
  • a certain distance apart, so that the time taken to travel from one road marking to the next can be worked out
Triangle showing distance over average speed times time.You may find this triangle helpful if you need to rearrange the equation for average speed

Example 3

Question

The speed limit on a road is 13.4 m/s (30 mph). Calculate the distance travelled by a car in 2 s at this speed.

average speed = distance ÷ time

Rearranging this equation:

distance = average speed × time

= 13.4 × 2 = 26.8 m

Question

Calculate the time taken for the car to travel 50 m at this speed.

Rearranging the equation:

time = distance ÷ average speed

= 50 ÷ 13.4 = 3.7 s