# Motion along a straight line

The trajectory of a point that moves in a linear motion is a straight line. An origin and a direction are set arbitrarily on the line, so that the motion of this point can be described by the coordinate (motion in one dimension).

The *variation* of position of the point with time defines the velocity; the *variation* of the velocity with time is defined as acceleration.

### Velocity[edit | edit source]

**Definition**: *Average velocity* is defined as the ratio between the displacement and the interval of time (in which the displacement occurs):