Distance refers to the movement of particles from one position to another in space, and the length of trajectory is called the distance that particles pass during this movement. Distance is a scalar, that is, a quantity without direction. Displacement and distance are two different physical quantities.

The calculation formula of distance is distance = time× speed, speed = distance/time, and time = distance/speed.

There are three situations involving the movement of two objects: opposite movement (encounter problem), same movement (chase problem) and opposite movement (separation problem). But to sum up, whether it is the movement of one object or the movement of two objects, whether it is the opposite movement, the same movement or the opposite movement, their characteristics are the same. Specifically, they reflect the same quantitative relationship, which can be summarized as: distance = speed × time.

The meaning of mathematical unit:

(1) the unit of the original quantity (refers to the smaller quantity in the original quantity, such as the distance is 3 hours, and the average distance per hour is a unit of distance. ) or the unit of a number can be converted into a smaller unit than "1", so there is a definition of fraction: divide unit one (or the whole "1") into several parts on average, indicating that one or more of them are fractions.

②. You can use "1" as the unit to redefine another quantity that is the same as the original quantity unit, and express it as a fraction. This score is often called the corresponding score of another quantity.

Generally, the method of generating scores is called segmentation method, and the method of generating scores is called ratio method. In the division method, "1" is in the molecular position, and in the ratio method, "1" is in the denominator position.