Question

A passenger in a car wants to check that the car’s speedometer is working correctly.
there are lamp posts on the side of the road.
The passenger knows that the lamp posts are 80 m apart.
Explain how the passenger can check that the speedometer is working correctly.

Answer 1

He can record time that car takes to pass between two neighboring lamp post. Let's denote this time with $t$. As we know, the distance between two neighboring lamp post is $h = 80m$. Thus, in order to find car's speed, we should calculate the following expression:

$$v = \dfrac{h}{t} = \dfrac{80}{t}\dfrac{m}{s}$$

In this way the passenger can verify the speedometer.