# Spatial orientation

`The lines r, set have the direction in common. The line segments PQ, RS and MN have the same direction`

When we study Physics, we come across different concepts, formulas, experiments, etc. We use measurement units, for example, to characterize well the physical quantities we study.

When we say that a body has a mass of 200, it is necessary to characterize this mass in kilograms or grams. Therefore, when we say that it has a mass of 200 g we have a better idea of the mass of the body. These physical quantities are called scalar quantities.

In other situations, there is a need for a better identification of the physical quantity. This is the case of physical quantities known as vector quantities. In these cases, the physical quantity needs an intensity (number followed by a unit of measurement) and its spatial orientation (direction and direction).

**Spatial orientation (direction and direction)**

We say that two lines have the *same direction* when they are *parallel* . Two or more line segments have the same **direction** when they are on the same line or when they are on parallel lines, that is, the direction of a line segment is the same as the direction of the line supporting that segment (figure). When this is not the case, the lines will have different directions.

Let’s consider a line segment *AB* . On this segment we can imagine two directions of travel: one from A to B, the other from B to A. Then we can consider two segments with different orientations: the oriented segment and the oriented segment .

The oriented segments MN and PQ, represented in the figure below, have the same direction (ie, the direction of the line r) and opposite directions; while CD and RS have the same direction and the same sense.

Notice that we naturally and intuitively adopt the following notations and nomenclature: given an oriented segment AB, A is the origin and B is the endpoint. See the figure below:

An oriented segment , where A coincides with B, is called a *null segment* . In reality, the null segment corresponds to a point and, as such, admits of any direction and sense.

In the figure below, the oriented segment has a length of AB, which is called a **module** and can be placed in any unit. The segment has unit length ** u** . The oriented segment has length 2u, so the modulus of the oriented segment is e . Likewise, we have: