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Speed camera

By David, 21 October 2014 Activity

How busy is the road near your school? do cars drive past too fast? Grab a stopwatch and sit yourself down – it’s time to become a human speed camera!

Text says distance from tree to lamp 20 m

Measure the distance between two objects on a road, and write it down.

hazard iconhazard iconSafety: This activity is about watching cars. Younger mathematicians should ask an adult to supervise. Protect yourself from the sun by wearing a hat, shirt, sunglasses and sunscreen

You will need

  • Two stopwatches
  • Clipboard and paper or an exercise book
  • Pen
  • Long tape measure or trundle wheel

Setting up

Car passes lamp post

When a car comes past the first object, start the timer.

  1. Go to the edge of a road outside your school.
  2. Find somewhere to sit near the road where you can see a long stretch of road.
  3. Choose an object near the road some distance from where you are sitting (but not too far) in one direction. Choose an object in the other direction. They could be street signs, trees or anything that is easy to spot.
  4. Carefully, with the long tape or trundle wheel and without walking on the road, measure the distance from one object to the other. You may need an adult to help. Record the distance (in metres) on your clipboard.
  5. Go back and sit where you can see both your objects.

    Car passes tree

    Stop the timer when the car passes the second object.

Beccoming a human speed camera

  1. Set a timer on one of your stopwatches for 10 minutes.
  2. Hold the other stopwatch and look along the road for a coming car. When a car passes the first object, start the timer. When it passes the second object, stop the timer.
  3. Record the time in seconds on your clipboard.
  4. Look along the road for another car. Time how long it takes to pass between the two objects and record that time too. Keep timing and recording cars until your 10 minutes are up.
  5. You have recorded a distance (in metres) and lots of times (in seconds). Each time is how long it took one car to cover the distance.
  6. To find out how fast each car was going, divide the distance travelled by the time it took.
    Timer reads 8.46 seconds

    Record the time in seconds on your clipboard.

    For example, if the distance was 40 metres and a car took 4 seconds, they were travelling at 10 metres per second.

  7. If you multiply the speed by 3600, you will find how many metres the car would travel in an hour. Ten metres per second is 36 000 metres per hour. Dividing that by 1000 will give the speed in kilometres per hour – ten metres per second is 36 kilometres per hour.
  8. Can your experiment answer these questions about the traffic outside your school?
  • Do cars break the speed limit outside the school?
  • Is it dangerous to cross the road outside the school?
  • What is the average speed of cars outside the school?

Analysing the data

  1. You have recorded a distance (in metres) and lots of times (in seconds). Each time is how long it took one car to cover the distance.
  2. To find out how fast each car was going, divide the distance travelled by the time it took. For example, if the distance was 40 metres and a car took 4 seconds, they were travelling at 10 metres per second.
  3. If you multiply the speed by 3600, you will find how many metres the car would travel in an hour. Ten metres per second is 36 000 metres per hour. Dividing that by 1000 will give the speed in kilometres per hour – ten metres per second is 36 kilometres per hour.
  4. Can your experiment answer these questions about the traffic outside your school?
    Do cars break the speed limit outside the school?
    Is it dangerous to cross the road outside the school?
    What is the average speed of cars outside the school?

What’s happening?

Safety is really important, and you might have some ideas about how to improve safety in your town. But to find out if your ideas will work, you need to take some measurements. When you finish this activity you will know a lot more about how fast people drive outside your school. And if you find people are being unsafe, you could ask the police to set up a real speed camera!

A well-designed experiment will often only answer one or two questions. Many problems, such as road safety, are complex, and can’t be answered up with just one experiment. To get a full understanding of the situation, many questions need to be answered, and each may need their own experiment.

It’s not always easy to keep your community safe, but maths will help you understand the dangers with more precision, and find good solutions.

More information

Read an article about average speed cameras in The Conversation
Why not try and measure the speed of your fidget spinner?

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