Speed is a scalar physical quantity that measures how quickly distance is covered. Average speed is defined as total distance travelled divided by the total elapsed time. Unlike velocity, speed has no direction and therefore does not indicate which way an object is moving. The SI unit of speed is metres per second, although kilometres per hour, miles per hour and other units are widely used.
An object can have a constant speed while its velocity changes if its direction changes, as occurs in uniform circular motion. Speed is therefore fundamentally different from velocity even though the two quantities are closely related. The distinction is important because distance is a scalar quantity while displacement is a vector quantity, leading to different definitions of average speed and average velocity.
Average speed and instantaneous speed must be distinguished. Average speed describes the entire distance travelled over a specified time interval, while instantaneous speed describes how fast an object is moving at a particular instant. Mathematically, instantaneous speed is the magnitude of instantaneous velocity. A car that travels 100 kilometres in two hours has an average speed of 50 kilometres per hour, but its instantaneous speed may vary widely during the journey because it may accelerate, brake, stop or change speed repeatedly. Average speed therefore does not imply that the object travelled at that speed continuously. Instantaneous speed can be measured using devices such as radar, laser speed measurement, wheel sensors or other systems that determine changes in position over short time intervals.
Speed differs from velocity because speed has magnitude only, while velocity has both magnitude and direction. Consider an object travelling around a circular track at constant speed. Its speed can remain unchanged while its velocity continually changes because the direction of motion changes. Since acceleration is defined as the rate of change of velocity, the object therefore accelerates even though its speed is constant. This is a common source of confusion. Another distinction concerns average quantities.
Average speed is total distance divided by total time, whereas average velocity is displacement divided by total time. If an object completes a complete circular lap and returns to its starting point, its displacement is zero and its average velocity is zero, but its average speed is positive because it travelled a nonzero distance.
Speed is commonly expressed in different units and can be converted using dimensional relationships. One metre per second corresponds to 3.6 kilometres per hour. A speed of 10 metres per second is therefore 36 kilometres per hour. In scientific calculations, SI units are generally preferred because they provide consistency with other physical quantities. Speed can also be expressed as a fraction of the speed of light in relativistic physics. The speed of light in vacuum is exactly 299792458 metres per second under the modern SI definition. Massive objects can approach but cannot reach this speed through ordinary acceleration because the required energy increases dramatically as their speed approaches the relativistic limit.
Speed appears in many physical contexts beyond ordinary travel. The thermal motion of particles is related statistically to temperature, although temperature is not simply identical to particle speed. Waves have characteristic propagation speeds determined by their physical medium or field properties. Sound travels at different speeds in different materials because its propagation depends on properties such as elasticity and density. Electromagnetic waves in vacuum propagate at the invariant speed of light. In fluids, flow speed describes the local velocity magnitude of the fluid, while weather systems and ocean currents are commonly described using velocities that include direction. Thus speed is a general kinematic quantity rather than a concept restricted to vehicles or visible moving objects.
Speed is not itself a measure of acceleration. An object can move rapidly with zero acceleration if its velocity is constant in an inertial reference frame. Conversely, an object can have zero instantaneous speed while having nonzero acceleration, such as a ball momentarily at rest at the highest point of an idealised vertical trajectory. At that instant its velocity and speed are zero, but gravitational acceleration remains approximately downward. Acceleration measures the rate of change of velocity, so it can occur through changes in speed, changes in direction or both. This distinction is important in analysing motion because speed alone does not contain enough information to determine acceleration.
In practical measurement, speed is often inferred rather than directly observed as an abstract quantity. A vehicle speedometer can estimate speed from wheel rotation, drivetrain measurements or other sensors. Radar systems use changes in electromagnetic wave frequency or phase to infer the speed component along the radar line of sight. Global navigation satellite systems determine position changes and can calculate velocity and speed from satellite signals, although the underlying measurements involve timing and relativistic corrections.
Laboratory experiments may determine speed by measuring position at successive times and calculating the rate of change. Measurement uncertainty affects the resulting speed, particularly when very small changes in position are divided by short time intervals. Accurate speed measurement therefore depends on both instrumentation and appropriate mathematical treatment of data.
Speed is limited by the physical context and theoretical framework being used. In classical mechanics, speeds can be treated as unbounded mathematically, although practical forces and energy supplies impose limits. In special relativity, the speed of light in vacuum is an invariant upper limit for the propagation of information and for the motion of massive objects. Objects with mass require increasingly large amounts of energy as their speed approaches the speed of light.
The speed of light is not simply another fast speed but a fundamental constant connecting space and time. Consequently, ordinary statements such as a spacecraft travelling at a particular fraction of light speed must be interpreted within relativistic kinematics. For everyday terrestrial motion, however, classical definitions of speed provide an accurate and straightforward description.
