Speed, distance and time

We know Speed = Total distance ÷ Total time. The same link can be turned around, depending on what you are given:

• To find distance: Distance = Speed × Time
• To find time: Time = Distance ÷ Speed

The book works both. Raghav's bus moves at 50 km/h and takes 2 h to reach the city. Distance = 50 km/h × 2 h = 100 km.

A train at 90 km/h covers 360 km. Time = 360 km ÷ 90 km/h = 4 h.

Before comparing two speeds, make sure they are in the same unit. A speed in m/s and a speed in km/h cannot be compared directly; change one into the other's unit first.

Now an important caution. In these examples we divided the total distance by the total time. But a bus does not go at exactly the same speed the whole way: it slows at a crossing, speeds up on an open road. So the speed we calculated is really the average speed. In this book, 'speed' means average speed.

Vehicles have two instruments for this:

• Speedometer: shows how fast the vehicle is going right now, in km/h.
• Odometer: shows the total distance travelled, in kilometres.