In kinematics, speed is a scalar quantity, because the speed of an object is the designation of the movement (the rate of change of its position). The Italian physicist Galileo Galilei defined speed as the distance covered per unit of time. In the International System of Units the speed is measured in meters per second. But in everyday life, the more commonly used unit of speed is kilometer per hour and miles per hour in the US and the UK. The knot is typically used in air and sea travel.