Position

The position of an object is described by telling three things about it:

1) The location of a reference point, called the origin. This is usually defined in a drawing of the experimental situation which shows the location of the origin.
2) The distance of the object from the origin.
3) The direction in which you must move from the origin in order to go to the object.

The distance and direction information is combined into the position vector, drawn here with an arrow above a letter. (The arrow reminds us that there is a direction associated with this algebraic variable.) Vectors are represented pictorially as arrows. The direction of the arrow represent s the direction of the vector. For position, the length of the arrow represents the distance from the origin.

When an object moves from one place to another, it is said to be "displaced." When it moves, the position of the object changes. The displacement vector is defined to be the vector which connects the old position to the new position, as sketched in the figure. Displacement differs from position in that no origin is needed to specify a displacement.

In the figure, the subscript "f" represents the final position, and the subscript "i" represents the initial position. The drawing represents the displacement "Delta R" associated with changing from R(initial) to R(final).

Since the displacement represents the difference between two positions, it is written algebraically as a subtraction. For more on adding and subtracting vectors, see the article on vector addition.

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