Position Vector

In physics, the position, the position vector or the location vector of a body with respect to a coordinate system is defined as the vector that links the location of the body with the origin of the coordinate system. In Cartesian coordinates, it is expressed as:

r=xi+yj+zk

Where:

  • r : Is the position vector
  • x, y, : Are the coordinates of the position vector
  • i, j, k : Are the unit vectors in the directions of axes OX, OY and OZ respectively.

The unit of measurement for position in the International System is meter [m]. Like all vectors, the position vector in physics has direction and magnitude (also known as size, modulus or length of the vector). The magnitude of the position vector is the distance of the body to the origin of the reference system. To calculate it you can use the following formula:

r=x2+y2+z2

For those problems where you are working in fewer dimensions, you can simplify the previous formula by eliminating unnecessary terms. This way, the position equation:

  • In two dimensions becomes r=xi+yj+zk=xi+yj ,and its magnitude r=x2+y2+z2=x2+y2, since z=0
  • In one dimension becomes r=xi+yj+zk=xi, and its magnitude r=x2+y2+z2=x2=x, since y=0 and z=0

We have represented a position vector in three dimensions (left) and another in two dimensions (right) in the following figure:

position vector in 3 and 2 dimensions

Experiment and Learn
 
Position vector and its magnitude

Slide the body within the coordinate system and observe how the magnitude is as as the coordinates and direction of its position vector change.
Notice that as the body approaches the origin of coordinates (0,0) the modulus of the vector, which is expressed as | r |, decreases. What happens if you move it away? , is reduced. What if you move it away?

NOTE: The dotted line is not usually drawn, we show it here so you can see more clearly the direction of the vector.

Formulas
r=   · i · j m
r=   m

Solved exercises worksheet

Here you can test what you have learned in this section.

What is the position vector?

difficulty

Find the position vector and its magnitude for the following points:

a) P1 (-1,2)
b) P2 (-3,4)

Formulas worksheet

Here is a full list of formulas for the section Position Vector. By understanding each equation, you will be able to solve any problem that you may encounter at this level.

Click on the icon   to export them to any compatible external program.

Modulus position vector in Cartesian coordinates in 3 dimensions

r=x2+y2+z2

Modulus position vector in Cartesian coordinates in 2 dimensions

r=x2+y2

Position vector in 2 dimensional Cartesian coordinates

r=xi+yj

Position vector in 3 dimensional Cartesian coordinates

r=xi+yj+zk

Related sections