(b) Figure 4 shows a car travelling on a motorway. A passenger wants to check the accuracy of the speedometer of the car.
The car is travelling at a constant speed.
The passenger has a stopwatch.
The lamp posts are 40 m apart.
lamp post
Figure 4
Describe how the passenger could determine the speed of the car as accurately
as possible.
(3)

To calculate anything - speed, acceleration, all that - we need data. The more data we have, and the more accurate that data is, the more accurate our calculations will be. To collect that data, we need to measure it somehow. To measure anything, we need tools and a method. Speed is a measure of distance over time, so we'll need tools for measuring time and distance, and a method for measuring each.

Conveniently, the lamp posts in this problem are equally spaced, and we can treat that spacing as our measuring stick. To measure speed, we'll need to bring time in somehow too, and that's where the stopwatch comes in. A good method might go like this:

Of course, you'll never find an *exact* amount, but the more data points you have, the better your approximation will become.