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.
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:
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).
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:
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
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