Describing Motion

To describe the position of an object we need to specify a reference point called the origin.

Motion along a straight line

The simplest type of motion is the motion along a straight line. Consider the motion of an object moving along a straight path. The objects starts its journey from O which is treated as its reference point. Let A, B and C represent the position of the object at different instants. At first, the objects moves through C and B and reaches A. Then it moves back along the same path and reaches through B.

The total path length covered by the object is OA + AC, that is 60 km + 35km = 95km. This is the distance covered by the object. The numerical value of a physical quantity is its magnitude.

Displacement:- The shortest distance measured from the initial to the final position of an object is known as the displacement. It is a vector quantity.

Displacement can be negative (-), positive (+) or zero. Displacement can be equal to distance or its lesser than distance.

In the above figure if objects starts from O and travel to point A and back to its original position O then its displacement is zero (0) and if objects reaches at C then displacement will be 25km.

Two different physical quantities-the distance and the displacement, are used to describe the overall motion of an object and to locate its final position with reference to its initial position.

A device that shows the distance travelled by automobiles. Such device is known as an odometer.