Speed is a measure of how fast an object moves, expressed as the distance it covers in a given interval of time. In physics speed is a scalar quantity: it has magnitude but no direction. When people speak loosely about how quickly something moves they usually mean speed, while the related vector quantity that includes direction is called velocity.
Definition and basic formula
A common way to express average speed is the total distance traveled divided by the elapsed time. Symbolically this is written v = d/t, meaning speed equals distance over time; in words, the average speed is found by dividing total path length by the time taken. This simple ratio is appropriate when motion is measured over finite intervals.
Average versus instantaneous speed
Average speed applies to a whole trip or interval. Instantaneous speed describes how fast an object is moving at a particular moment and is defined mathematically as the limit of the average speed for ever-smaller time intervals. In calculus terms, instantaneous speed is the magnitude of the derivative of position with respect to time. Instruments and measurements often approximate instantaneous values by sampling position rapidly.
Units and common measurements
Speed is expressed in units of length per time. Common units include metres per second (m/s), kilometres per hour (km/h) and miles per hour (mph). Typical methods for measuring speed include mechanical and electronic speedometers in vehicles, radar and lidar instruments, Doppler shift techniques, and timing over a known distance. Practical examples range from a runner's pace to orbital speeds of satellites.
Applications, limits and notable facts
Speed is central in transportation, engineering, sports, and astronomy. In everyday contexts it determines travel times and safety limits; in science it governs dynamics, energy, and kinematics. In modern physics there is an ultimate speed limit: no information or material object can exceed the speed of light in vacuum, approximately 3×10^8 m/s, a result with consequences for time and simultaneity at high speeds.
History and important distinctions
Concepts of speed have evolved from early qualitative descriptions of motion through classical mechanics to precise modern definitions using calculus and relativity. It is important to distinguish speed (a scalar) from velocity (a vector) and from related everyday measures such as pace (time per unit distance). For further foundational reading see general references on kinematics and measurement practice: basic kinematics.