we represent the trajectory t of a particle. In scalar kinematics we determine the position of the particle by its abscissa (or space) S. In vector kinematics we determine the position through its vector 
. This vector has its origin at a point O (chosen at random) and ends at the point where the particle is.
we represent the positions of the particle at times t 1 and t 2 (with t 2 > t 1 ). In scalar kinematics we define the space variation Δs by Δs = S 2 – S 1 . In vector kinematics we define vector displacement.jpg)
in this time interval by:
= 
_ 
That is, the vector displacement (or displacement vector ) is the vector represented by the oriented segment whose origin is the endpoint of and whose endpoint is the endpoint .
|∆s| > | |
When
|∆s| = | |
the trajectory is rectilinear or Δs = 0. Thus, in general we have:
|∆s| ≥ | |
The average speed is defined by:
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The average velocity of a particle is defined by:
As Δt > 0, the values of e 
must have the same direction and the same sense, as long as ≠ 0 . We have seen that, in general, |∆s| ≥ | | , thus, dividing by Δt the two members of this inequality, we have:
