4.1 Speed
Summary: It feels fast going downhill on a bike. Even going downhill on a bike, you could not travel as fast as the cheetah in Figure 1. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
Think back
- Give the unit of distance we use in science.
- Give the unit of time we use in science.
- Give the steps for writing out a calculation.
Key idea Speed describes how far an object travels in a particular time. We measure speed in metres per second (m/s) in science.
Key words speed, metres per second (m/s), instantaneous speed, average speed, relative motion
It feels fast going downhill on a bike. Even going downhill on a bike, you could not travel as fast as the cheetah in Figure 1**.**
Figure 1A cheetah can travel faster than a car.
How fast?
Speed is a measure of how far something travels in a particular time. In science, we measure speed in metres per second (m/s). Car speed and speed limits are measured in miles per hour (mph) or kilometres per hour (km/h). A traffic speed camera, like the one in Figure 2, makes these measurements. A very fast car can move at over 300 km/h. However, when you walk to school, you are probably walking at 5 km/h or about 1 m/s. Table 1 shows some speeds in different units.
Figure 2A speed camera calculates how long it takes a car to travel a certain distance by taking a photo at the start of the distance and another at the end. Table 1 Different speeds in different units of measurement. Speed in m/s
Speed in km/h
Speed in mph
Walking quickly
1.7
6.1
3.8
Sprinting (running quickly)
10
36
22
Typical speed limit
14
50
31
Cheetah
33
119
74
Aeroplane flying speed
255
918
570
Sound in air
330
1190
738
Light in air
300000000
10000000000
670000000
A State the unit of speed that we use in science.
How do you calculate speed?
To find the speed of an object moving at a steady (constant) speed, you need to measure the time it takes to travel between two points. You work out the speed from the distance travelled divided by the time taken:
A long-distance runner runs part of their race at a steady speed. It takes them 20 seconds to run 100 m.
When you are using an equation, it is helpful to write it out like this. If you write the units of distance and time in the equation, you will have the correct units for the speed.
Maths skills A marathon runner runs 42 km in 2 hours 30 minutes, or 2.5 hours. Calculate the average speed of the runner in km/h.
Instantaneous or average speed?
A runner will take many seconds, minutes, or hours to run a race. They do not run at exactly the same speed throughout the race.
The speed that the runner is travelling at any time during the race is the instantaneous speed. This is the speed that you see on the speedometer in a car. You can work out the average speed by dividing the total distance by the total time that it took to run the race. This average speed makes it easier to compare how fast different people, boats, or cars travel.
B A ball falls and hits the ground. State where its instantaneous speed is fastest.
What is relative motion?
Figure 3 shows an example of relative motion. Relative motion means how fast one thing is travelling compared with another. For example, being on a moving train looking at a stationary (still) one feels the same as being on a stationary train looking at a moving one. Speed is relative. If two cars are moving at the same speed in the same direction, their relative speed is zero. We usually measure speed relative to the ground.
Figure 3Relative speeds. Full text alternative to the content
C Look at Figure 3. State the speed of the red car relative to the ground.
Stretch zone
- Copy and complete the sentences. To calculate speed, you need to know the distance travelled/method of travel and the temperature/time taken. To calculate average speed, you divide the average distance/total distance by the average time/total time. The instantaneous/relative speed is the speed at any one time. You measure instantaneous/ relative speed against a stationary object.
- A runner runs 100m in 12.5 seconds. Calculate their average speed.
Stretch zone
- A car is travelling east at 50 km/h. On the same road, another car is travelling west at 20km/h. The cars are moving away from each other. Describe their relative motion.
- Change the subject of the speed equation to find time.Calculate the time in seconds it would take sound and light to reach you from a lightning strike that is 6 km away. Use the data in Table 1 to help you.