Suppose you have a toy car and you want to know how fast it goes when you roll it down the hallway. You don’t have anything fancy like a radar gun… so how do you do it?

Ask yourself; what do we know about the hallway? Well, if we measure from one doorway to the next, we find that the hallway is about 10 meters long. Great.

Using a stopwatch, you can see how long it takes for the car to go from the first doorway to the next. Doing it a few times (say, 10 times), recording the time each time, and then summing those values and dividing by 10 (the number of times we did it), we arrive at an average time of 6.32 seconds. Let’s assume we are pushing the car along with about the same amount of force each time, ok?

There’s a formula in physics for finding the velocity of an object (ignoring things like wind resistance, friction between the tires and the floor, and friction between the tires and the car itself). velocity = distance traveled divided by the time it takes to get there, or v=d/t. We have discovered that the distance of the hallway is about 10 meters and the time it takes the car to travel that distance is an average of 6.32 seconds. 10m/6.32s = 1.58m/s, or 1.58 meters per second.

While correct, this number isn’t super useful to us… how fast is that in kilometres per hour? That’s how car speed is measured so it’s easier to understand.

There are 1000 meters in a kilometre, and there happen to be 3600 seconds in an hour (60 seconds per minute, 60 minutes per hour; 60*60-3600). To find how fast this car is moving in kilometres per hour:

Divide 1.58 by 1000 and then multiply by 3600. The result? About 5.67 kilometres per hour. Another way is to start with knowing that 1 meter per second is 3.6 kilometres per hour (1/1000*3600). Take our velocity of 1.58m/s and multiply it by 3.6. The result is about 5.68km/h

Science!