Final answer:
Speed is the scalar quantity that measures how fast a body is moving, and velocity is the vector quantity that includes both speed and direction. These are calculated by dividing the distance or displacement by the time interval over which the motion occurs, with time being a fundamental measurable quantity representing an interval of change.
Step-by-step explanation:
The physical quantity that informs us about how fast or slow a body is moving is known as speed and, when direction is included, it is known as velocity. To calculate these quantities in kinematics, we consider the time variable, which allows us to determine the body's position over time. Specifically, the rate at which the position changes with time is a defining factor for speed and velocity.
To define a physical quantity like speed or velocity, we can specify the method of measurement or calculate it from other measurements. For example, the average speed is calculated by taking the total distance traveled and dividing it by the time taken to travel that distance. Conversely, average velocity is a vector quantity that not only considers the magnitude of speed but also its direction, and it's calculated by dividing the displacement by the time interval over which the displacement occurs.
As such, time is a fundamental physical quantity measured by the interval over which change, such as movement, occurs. Without the progression of time, no change can be observed, and therefore, no motion can be quantified.