hraði

Hraði is the Icelandic word for speed. In broad terms, it denotes how quickly an object covers distance over time. In physics, speed is often treated as a scalar quantity that only has magnitude, distinguishing it from velocity, which also specifies a direction.

Average speed is defined as total distance traveled divided by the elapsed time. Instantaneous speed is the speed at a specific moment and is mathematically the derivative of position with respect to time. Units commonly used to measure speed include meters per second (m/s) in scientific contexts and kilometers per hour (km/h) or miles per hour (mph) in everyday life. The speed of an object is influenced by forces acting on it, via Newton's laws; acceleration changes speed when the force has a component along the direction of motion.

In practical terms, measuring speed enables navigation, traffic safety, sports analysis, and physical experiments. Devices such as speedometers, radar guns, and laser-based rangefinders are used to determine speed in various applications. In wave phenomena, the term speed applies to the propagation velocity of waves, such as sound speed in air or light speed in vacuum, which is approximately 299,792,458 m/s and serves as a fundamental physical constant.

The concept of speed intersects with related ideas like velocity, momentum, and kinetic energy. Historical developments in kinematics, from early observations to Newtonian mechanics, formalized precise definitions and methods for calculating and predicting speed in moving bodies.