What is the SI Unit of Speed?

The official International System (SI) unit of Speed is meters per second (m/s) or m s⁻¹.

Why? Because the SI base unit for distance is the Meter (m), and the SI base unit for time is the Second (s). By dividing distance by time, we get meters/second.

While scientists use m/s, it is not practical for everyday life (like driving). Therefore, other larger or smaller units are widely used:

1. Kilometers per hour (km/h or kmph): The standard unit used for the speed of cars, trains, and airplanes worldwide.
2. Miles per hour (mph): The standard vehicle speed unit used strictly in the United States and the UK.
3. Centimeters per second (cm/s): Used in chemistry and biology to measure very slow things (like the speed of an ant or a chemical reaction).

Physics exams will frequently ask you to convert a car's speed from km/h into m/s before solving a numerical problem.

The Trick:

• To convert km/h to m/s: Multiply the number by (5 / 18).
• To convert m/s to km/h: Multiply the number by (18 / 5).

Example: A car is moving at 72 km/h. What is its speed in m/s? Calculation: 72 × (5/18) = 4 × 5 = 20 m/s.