Speed

Speed is a physical measure of the distance that an object travels in a given time. It is a scalar quantity—it has only magnitude, with no direction. In the international system of units (SI), it is measured in metres per second (m/s), and also in kilometres per hour (km/h) or knots when referring to maritime and aviation contexts.

The basic formula is: speed = distance / time. An important distinction exists between average speed (measured over an entire journey) and instantaneous speed (at a particular moment, approximated by taking an infinitesimally small time interval).

Speed and velocity

English distinguishes between 'speed' and 'velocity'. Speed is a scalar—magnitude only. Velocity is a vector—magnitude and direction. An object moving in a circle at constant speed has changing velocity, because the direction is constantly changing. The rate of change of velocity over time is acceleration.

The speed of light

According to Albert Einstein's theory of special relativity, the speed of light in a vacuum is the upper limit at which information or matter can travel. It is approximately 299,792,458 metres per second, now defined as a fixed quantity on which the definition of the metre is based. As an object's speed approaches this limit, relativistic effects—such as time dilation—become observable.

Orbital speed and escape velocity

Orbital speed is the speed required for an object to remain in orbit around a large body with gravitational pull. Escape velocity is the minimum speed at which an object can leave a body's gravitational influence without falling back. Both quantities depend on the mass of the body and the distance from its centre.

Measurement

Speed is measured using various instruments: speedometers in vehicles, radar and laser guns used by traffic police, and the Doppler effect for measuring the speed of stars by observing shifts in their light. The speed of sound in air near sea level is approximately 343 m/s at 20 degrees Celsius, but varies with temperature and humidity.