2.4 Distance and Displacement
We know that motion is the action of an object going from one place to another or changing position. The length between the original and final positions may be measured in two ways: either as distance or as displacement.
Distance is the length of the actual path of the motion.
Let a person be travelling from Lahore to Multan in a car. On reaching Multan, he reads the speedometer and notices that he has travelled a distance of 320 km. It is the distance travelled by that person. Obviously, it is not the shortest distance from Lahore to Multan, as the car took many turns along the way and did not travel along a straight line.
Displacement of an object is a vector quantity whose magnitude is the shortest distance between the initial and final positions of the motion, and its direction is from the initial position to the final position. We can also call this the change in position.
Note that displacement is a vector quantity, whereas distance is a scalar quantity. The SI unit for displacement is the same as that of distance.
Suppose a car travels from position A to B. The curved line is the actual path followed by the car. The total distance covered by the car will be equal to the length of the curved line AB. The displacement d is the straight line AB directed from A to B, as indicated by the arrow head.