Road traffic must obey the speed limit, for the safety of everyone. Speed cameras may be used to check that drivers are not going too fast.
A speed camera stands at the side of the road. It measures the speed of cach vehicle as it passes. It takes a photograph of any car which is speeding (going faster than the speed limit). Then the driver can be identified from the registration number.
One type of camera works like this:
• There are two detector strips in the road. They are a known distance apart.
• The camera detects the car as it passes over each strip. A timer in the camera measures the time taken by the car to travel from one strip to the next.
• A mini-computer calculates the speed of the car. If the car is speeding, the camera takes a photograph of it.

A speed camera calculates the speed of each passing car.

Light gates

In the laboratory you can use light gates to measure the speed of a moving object. A light gate is similar to a speed camera on the road.
The picture shows one way to use two light gates to measure the speed of a moving trolley.

Each light gate has an invisible beam of infrared light. When anything breaks the beam, the light gate sends a pulse of electricity to the timer.
• One light gate is connected to the START terminal of an electronic timer.
• The other light gate is connected to the STOP terminal.
The picture shows a trolley running past the light gates. It distance breaks the first beam; this starts the timer. Then it breaks the second beam; this stops the timer.
The timer shows how long it took for the trolley to move between the two light gates. We can measure the distance between the two light gates. Then we can calculate the trolley's average speed:

$speed = \frac{{distance}}{{time}}$

Fractions of a second

Light gates are useful because they can measure very short time intervals - less than a second. You cannot use a stopwatch to measure times less than one second. Light gates can be connected to a computer instead of a timer. Then the computer can calculate the speed of the moving object. Before it can do this, you need to tell it the distance between the two light gates.