In order to describe the motion of an object we must be able to specify its position. This is done by stating how far the object is and in which direction it lies from a reference point. This is referred to as the object"s displacement (s). Displacement is an example of a vector quantity. A vector quantity has both magnitude and direction. Vectors can be represented with arrows drawn to scale. The length of the arrow represents magnitude and the direction is shown by which way the arrow is pointing. e.g for displacement the length of the arrow represents the straight line distance of the object from the reference point in the direction that the arrow is pointing
The change in displacement Δs for an object which has moved is the straight line distance from its initial position to its final position.(The symbol Δ means "a change in".)
The first diagram below illustrates change in displacement
The objects initial displacement is subtracted its final displacement (both measured from the same reference point)
ΔS = S2 - S1
Compare this with the second diagram. In this case the reference point has been chosen to be the position the object started from and the diagram
shows the objects final displacement S from this reference position. By comparison with the first diagram it can be seen that in this
case ΔS = S are equivalent
(In summary: when we use the starting position of the object as the reference point for measuring displacement ΔS = S)
(Note : The above simplification can not always be applied e.g. we may have to consider several different periods of an object's motion using the same reference point for all measurements. In this case each period of the objects motion will start from different locations)
