Average speed can be calculated from the distance travelled and the time taken. The gradient of a distance-time graph is equal to the speed. Relative motion takes into account speed and direction.

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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:

You can calculate average speed using this equation:

average speed = distance ÷ time

- 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).

- 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

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

- 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