Speed
Speed tells you how much distance you cover per unit of time. With v=
You can start right here: nothing needed first.
Why
A De Lijn tram travels at 30 km/h, a top sprinter runs at 10 m/s. Which one is faster?
You cannot tell straight away, because the two speeds are in different units. Kilometres per hour and metres per second are both a distance divided by a time, but with different measures.
Convert 10 m/s and you get 10×
See it
Slide the distance and the time and press start. Read the speed in m/s and in km/h, and compare the cyclist with the shadow walking alongside.
- 5 km/h walking
- 18 km/h cycling
- 30 km/h a tram
- 50 km/h a car in town
Description of the picture
A straight track with a cyclist who covers 600 m in 120 s. Below it the speed is shown: 5 m/s, which is 18 km/h. A see-through walker at 5 km/h has only reached about 167 m at the same moment.
Get it
Speed tells you how quickly you change position: how many metres or kilometres you cover in one second or in one hour. Whoever covers more distance in the same time is faster.
In science we call the distance s, the time t and the speed v (from the Latin velocitas). Speed is distance divided by time: v=
You can write speed in different units. On the road we use kilometres per hour (km/h), in science often metres per second (m/s). An hour has 3600 seconds, so 1 m/s is 3600 m per hour, or 3.6 km/h. From m/s to km/h you multiply by 3.6, from km/h to m/s you divide by 3.6.
Hardly anyone travels at the same speed all the time. You brake before a junction and speed up again. That is why we often work with the average speed: the total distance divided by the total time, stops included.
v=
Worked example
Lotte cycles to school. She covers 9 km in 30 minutes. What is her average speed, in km/h and in m/s?
Write the time in hours, because you want km/h.
30min=
0.5h Divide the distance by the time. The answer is in km/h.
v=
9: 0.5= 18 From km/h to m/s you divide by 3.6. The answer is in m/s.
18:
3.6= 5 Check: in 1 s she covers 5 m, so in 1800 s (30 minutes) 5×
1800= 9000 m. That is 9 km, so it adds up. - Answer
Lotte cycles at an average of 18 km/h, which is 5 m/s.
Do it
Pick a level. You get a new exercise every time, checked straight away.
Misconceptions
Wrong: You first travel at 10 km/h and then at 30 km/h, so your average speed is 20 km/h.
That is only true if you spend the same time at each speed. If you cover stretches of the same length, you spend much longer going slowly than going fast.
Right: Divide the total distance by the total time. Twice 10 km: 1 hour plus 20 minutes, so 20 km in 4/3 hours. That is 15 km/h.
Wrong: To convert km/h into m/s, you multiply by 3.6.
One metre per second is faster than one kilometre per hour. So a speed in m/s is a smaller number than the same speed in km/h.
Right: From km/h to m/s you divide by 3.6: 36 km/h is 10 m/s. From m/s to km/h you multiply by 3.6.
Wrong: Someone driving at 50 km/h drives 50 km.
A speed is not a distance. It tells you how far you would go if you kept that speed for a whole hour.
Right: At 50 km/h you cover 5 km in 6 minutes, a tenth of an hour. The distance also depends on the time: s=
v× t.
Try it at home
How fast does a marble roll?
What you need
- a marble or a toy car
- a tape measure
- sticky tape to mark the start and finish
- a mobile phone with a stopwatch
- a plank or a piece of cardboard and a few books for a small ramp
- pen and paper
How to do it
- Stick a start line on the floor and a finish line exactly 2 metres further on.
- Lean the plank against a few books, just before the start line. That way the marble is already rolling when it crosses the start line.
- Let go of the marble at the top of the ramp. Start the stopwatch when it rolls over the start line and stop it at the finish.
- Do this three times and take the average of the three times.
- Work out the speed with v=
s/t. If it rolls 2 m in 1.6 s, for example, then v= 2: 1.6= 1.25 m/s. Times 3.6 gives 4.5 km/h. - Add a book so the ramp gets steeper and measure again.
Safely: Work on a clear floor where nobody can trip over the plank. Pick up the marbles afterwards so nobody slips on them, and keep them away from small children.
What do you see, and why?
You are measuring the average speed over 2 metres. A steeper ramp gives a higher speed. Your reaction time with the stopwatch is about two tenths of a second, which is why you measure more than once.
In real life
- Many roads in Flanders have average speed checks. One camera records when you pass the start, a second camera when you pass the end. Distance divided by time gives your average speed.
- On the motorway in Belgium you may drive at 120 km/h at most. That is about 33 m/s: in one second a car travels a third of the length of a football pitch.
- A De Lijn tram often stops at tram stops and traffic lights. Its average speed is therefore much lower than its speed between two stops.
- In a 30 zone around a school, a car drives at 30 km/h at most, about 8 m/s. That gives the driver more time to brake.
History
On 5 May 1835 the first passenger train on mainland Europe ran in Belgium, between Brussels and Mechelen. Travelling became much faster than by horse and carriage. After that, trains had to run on time, and for that everyone needed to be good at calculating with distance, time and speed.
At the table
How far is it from home to school, and how long does it take you? What is your average speed then, by bike and by car?
A question to talk about together, for parents and children. There is no wrong answer.