Speed is a scalar quantity that describes how rapidly an object covers distance or, more precisely, the magnitude of its instantaneous velocity. Average speed is total distance travelled divided by total elapsed time, while instantaneous speed is the magnitude of instantaneous velocity. The SI unit of speed is the metre per second, although kilometres per hour, miles per hour and other units are common in practical contexts. Because speed is scalar, it does not specify direction. A car travelling at 50 km/h east and another travelling at 50 km/h west have the same speed but different velocities. Speed is therefore simpler than velocity in one respect, but it cannot by itself describe the complete state of motion of an object.
Average speed must be distinguished from the average of instantaneous speed values and from average velocity. If a journey consists of different stages, total average speed is calculated from the complete distance divided by the complete elapsed time, including relevant periods of stopping if the definition concerns the entire journey. It is not generally correct to take the arithmetic mean of two speeds unless the time intervals or other conditions justify that operation. Similarly, a round trip can have a positive average speed but zero average velocity because the net displacement is zero. These distinctions are important in transport calculations, athletic performance analysis, experimental measurements and interpretation of motion graphs.
Instantaneous speed describes the magnitude of motion at a particular instant. A vehicle's speedometer attempts to provide a measurement related to instantaneous speed, although the actual sensor and processing method may involve a finite measurement interval. In calculus, instantaneous speed is the magnitude of the velocity vector at that instant. For one-dimensional motion, if velocity is represented as a signed quantity, speed corresponds to its absolute value. In multidimensional motion, speed is calculated from the magnitude of the velocity vector, such as the square root of the sum of the squared velocity components. This distinction allows an object to change direction without changing speed while still undergoing acceleration.
Speed is often controlled or constrained by physical, engineering and regulatory factors. A vehicle's attainable speed depends on engine or motor output, gearing, aerodynamic drag, rolling resistance, gradient, available traction and mass distribution. At high speeds, aerodynamic drag can become a dominant opposing force and often increases approximately with the square of speed in common regimes, causing the power required to overcome drag to increase approximately with the cube of speed. Tyres, road conditions and braking systems also impose practical limits. In aviation, speed may be described using several distinct quantities, including true airspeed, indicated airspeed, calibrated airspeed and groundspeed, because air movement, atmospheric density and measurement conventions affect the meaning of a numerical speed.
Speed is not equivalent to acceleration. An object can move at high speed with zero acceleration if its velocity remains constant in a straight line, while an object can have zero speed at an instant while possessing non-zero acceleration. An object thrown vertically upwards slows as it rises because its velocity opposes gravitational acceleration, reaches zero instantaneous velocity at the highest point, and then increases in downward speed during the subsequent fall. In circular motion, an object can maintain constant speed while accelerating continuously because its velocity direction changes. These examples demonstrate why speed alone is insufficient for determining the forces acting on an object or predicting its future motion.
In relativistic physics, speed has an especially important role because the speed of light in vacuum is invariant for all inertial observers. Classical mechanics permits velocities to be added using simple vector addition, but special relativity replaces this with relativistic velocity transformation rules when speeds approach the speed of light. Massive objects cannot be accelerated to the speed of light because the required energy increases without bound as their speed approaches c within special relativity. The distinction between phase velocity, group velocity and signal propagation speed also becomes relevant in wave physics, meaning that the word speed can have more specialised meanings depending on the physical context.
Speed is measured through numerous technologies, including radar, lidar, Doppler sensors, optical encoders, wheel rotation sensors, GPS systems and timing gates. In road transport, speed measurement can be direct or inferred from rotational or positional information. Radar speed measurement commonly relies on Doppler frequency shifts, while timing gates calculate speed from a known distance and measured travel time. Measurement uncertainty depends on sensor accuracy, calibration, timing precision, environmental conditions and the mathematical model used. Units also matter: 1 m/s corresponds to 3.6 km/h, so numerical values cannot be compared meaningfully without accounting for unit conversion.
Common misconceptions include assuming that speed and velocity are synonyms, that a higher speed always means greater acceleration, and that average speed can always be obtained by simply averaging reported speeds. Speed is a magnitude and contains no directional information, whereas velocity is directional. A vehicle can have substantial speed while experiencing no acceleration, and a vehicle can have zero instantaneous speed while experiencing strong acceleration. Average speed is based on total distance and total time, not necessarily the arithmetic mean of individual speed readings. Understanding these distinctions makes speed a precise and useful quantity rather than merely a general description of how fast something appears to move.
