1How fast?
Speed tells you how far something travels in each second. A speed of 5 metres per second (5 m/s) means 5 metres every second.
speedmetres per second
Lesson 18.1 notes
Speed
Speed = distance ÷ time, working out speed, distance and time, and measuring speed.
2The speed equation
To work out a speed, divide the distance by the time: speed = distance ÷ time. A cyclist riding 600 m in 120 s goes at 600 ÷ 120 = 5 m/s.
distancetime
3Rearranging the equation
Cover the one you want in the triangle to see how to work it out: distance = speed × time and time = distance ÷ speed.
formula triangle
4Further isn't always faster
To compare speeds, work each one out. 200 m in 40 s is 5 m/s, but 100 m in 10 s is 10 m/s, so the shorter run was faster.
5Measuring speed
Measure the distance with a tape measure and time the journey with a stopwatch, then divide. Walking 10 m in 8 s is 10 ÷ 8 = 1.25 m/s.
stopwatch
6Light gates
For short times, use light gates and a timer. They start and stop the timing by themselves, so there is no human reaction time to spoil it.
light gatereaction time
7Typical speeds
Walking is about 1.5 m/s, cycling about 6 m/s and a car in town about 13 m/s. Sound travels at about 340 m/s in air, and light at about 300 000 000 m/s.
Explore: Speed calculator
Choose what to work out, then set the other two values. Watch the runner cover the distance and see the working.
Common mistake: Further doesn't always mean faster
A runner who goes further isn't always faster: it depends on how long it took. 200 m in 40 s is 5 m/s, slower than 100 m in 10 s, which is 10 m/s. Always divide the distance by the time.
Remember
- speed = distance ÷ time. In science, speed is measured in metres per second (m/s).
- distance = speed × time, and time = distance ÷ speed.
- Light gates time short events more accurately than a stopwatch, because there is no human reaction time.
Going further at GCSE
GCSE: Distance, speed and velocity → GCSE: Forces revision →