Fasthastighet

Fasthastighet is a term used in Swedish to describe a speed that does not change with time. The word is formed from fast, meaning fixed or steady, and hastighet, meaning speed. In contemporary physics, the preferred term is konstant hastighet (constant speed) or uniform hastighet, but fasthastighet appears in some educational resources as a synonym for a constant speed.

Different from the notion of constant velocity, fasthastighet focuses on the magnitude of motion remaining unchanged. If the object travels in a straight line with fixed speed, its velocity is constant. If the direction changes while the speed remains the same—such as in circular motion—the speed is constant but the velocity changes due to shifting direction, and acceleration is present.

Mathematically, constant speed means v = |v| is constant. Acceleration a equals dv/dt; it vanishes only if the velocity vector is also constant. In a curved path with constant speed, the acceleration points toward the centre of curvature with magnitude v^2/r.

Examples include a car cruising on a highway with cruise control, or a satellite moving at nearly constant orbital speed. Fasthastighet is a useful simplifying assumption in problems where speed remains the same while other quantities such as direction or orientation change.

See also: Uniform motion, Constant velocity, Velocity, Acceleration.