## Definition

The following is just a personal point of view.

Individual believes that the direction of motion is the movement trend of the object, it can have two understandings:

1. Object instant (a certain time t or a location X) Motion direction.

2. The direction of motion within a certain time interval Δt.

The motion direction of the former is the direction of the speed (instantaneous speed) of the object, which is an instant movement trend of the object; the latter movement direction is the direction in time ΔT, it is equivalent The displacement direction with the object. The instantaneous speed is the first order derivative of the position ** R ** (in the direction from the coordinate origin pointing to a certain position): ** r ', (displacement δ r = r2 ) means that it is the change rate of the bit arbitrary, and the direction is not necessarily consistent with the direction of the displacement (the ratio and acceleration Speed relationship). Instantaneous velocity: v = r ' = d r / dt, the moment displacement infinitesimal d instantaneous direction of motion of the object r The direction is consistent. For the movement direction of the object in ΔT, it is consistent with the displacement, which can also be obtained from the formula V = δ r / δ t t It is seen in / b>. **

Thus "The direction of the speed is the direction of the movement of the object" is convincing, but we need to use a different "motion direction" with its corresponding "speed when we understand this sentence." Direction "to understand: The direction of movement in the moment is equivalent to the direction of the instantaneous speed, and the direction of movement within a period of time is equivalent to the direction of the average speed during this period (summary: the direction of motion is the speed direction).