Speed Trap

Is the car speeding? Time it between two light gates, work out its speed and watch the distance-time graph draw itself.

How the car moves
Time
0.00 s
Distance
0.0 m
Speed now
15.0 m/s
Gate 1 → Gate 2
Waiting
Average speed
–
Average in km/h
–

The maths, live

Distance d = v × td = 15 × 0.00 = 0.0 m
Speed = distance ÷ timev = 40 m ÷ (time between gates)
In km/h multiply by 3.6Wait for gate 2

At a steady speed the graph is a straight line. What shape do you get when the car speeds up? Is the gate speed the same as its speed at gate 2?

How it works

A light gate is a beam of light across the road. When the car breaks the beam, a timer starts or stops. Divide the distance between the gates by the time between them and you get the average speed: speed = distance ÷ time.

On a distance-time graph, a steady speed makes a straight line, and the steeper the line, the faster the car. A car that is speeding up covers more distance every second, so its line curves upwards. Average speed cameras on real roads work in exactly this way.

Try this

  1. Drive at a steady 15 m/s. Is the car over the 50 km/h limit?
  2. Switch to speeding up. Is the car's speed at gate 2 the same as the average speed between the gates?
  3. Move the gates closer together. Does the average speed get closer to the speed at the gates?