Vector r, called position vector, is a directed line segment used to represent the spatial position of a moving particle (starting from the reference point and ending at the particle position).

The modulus of vector R is the length of position vector, that is, the square root of the sum of squares of xyz coordinates of position vector;

Scalar r is equivalent to the modulus of vector r;

The differential of vector r to time t is equal to instantaneous velocity (vector), and the modulus represents instantaneous velocity (scalar);

The differential of scalar r to time t is equal to the rate of change of position vector module, which can be considered as equal to the instantaneous rate under one-dimensional motion.

Furthermore, there is also the statement of radial velocity, which generally refers to the velocity component of the object velocity in the observer's line of sight, that is, the projection of the velocity vector in the line of sight. Therefore, there is also the saying of apparent speed, that is, the speed of an object or celestial body in the direction of the observer's line of sight.

Generally, it refers to the velocity component of the object's motion velocity in the observer's line of sight, that is, the projection of the velocity vector in the line of sight.