Speed: Measuring How Fast a Motion Is

Definition of speed

Speed measures how fast an object moves. It is calculated by dividing the distance traveled by the duration of the trip:

v = d / t

where v is the speed, d is the distance traveled, and t is the duration of the trip.

Units of speed

In physics, the legal unit of speed is the meter per second (m/s). In everyday life, the kilometer per hour (km/h) is often used.

Situation	Approximate speed
Walking	about 1.4 m/s, i.e. 5 km/h
Cycling	about 5 m/s, i.e. 18 km/h
Car on the highway	about 36 m/s, i.e. 130 km/h

Converting km/h to m/s

To convert a speed from km/h to m/s, divide by 3.6. To convert from m/s to km/h, multiply by 3.6.

Example: a car travels at 90 km/h. Its speed in m/s is 90 / 3.6 = 25 m/s.

Common pitfall to avoid

Be careful not to mix units in a calculation. If the distance is in meters and the time is in hours, you must first convert the time to seconds (1 hour = 3600 s) before calculating v = d/t, otherwise the result obtained makes no physical sense.