Position and Displacement

To describe an object's motion, its position needs to be specified relative to a convenient frame of reference. An arbitrary set of axes that describes an object's position and motion is known as a frame of reference.

Generally, Earth or stationary objects on Earth are used as frames of reference. For example, an airplane's position during its takeoff could be described with respect to Earth as a whole or the airport's control tower. To describe an object's position along a one-dimensional motion, we often use the variable x.

Now, suppose an object moves relative to a frame of reference—for example, a car moves to the left relative to a building, then the car's position changes. This change in position is called displacement.

However, the numerical value of x along a straight line defines an object's position where it might be located. Any change in position along this line is indicated by displacement, where Δx is displacement and is calculated by subtracting the initial position from the final position. It is a vector quantity since it indicates direction, and, depending on the choice of positive direction, it can be either positive or negative.