Fig. 2-1 The KCR railway from University Station to Tai Po Market station.

The distance travelled by a vehicle is the length of the path it covers regardless of direction. When we say that a KCR train has travelled a distance of about 6.7 km from University Station to Tai Po Market Station, we mean that the train has moved 6.7 km on the railway (Fig. 2-1). However, the railway is curved. In fact, if we measure their separation by drawing a straight line on the map, we will find that Tai Po Market Station is only 5.4 km from University Station, roughly in the North-West direction. This is the displacement of the train. The displacement tells us how far an object is from its starting position and in what direction. Displacement is therefore different from distance. To fully specify a displacement, we must specify both the straight-line separation and the direction of the destination from the starting position.

In physics, quantities that are described only by size (magnitude) are called scalars. Examples of scalars are distance, speed, mass, temperature and volume, etc. Quantities that are described by both size (magnitude) and direction are called vectors. Examples of vectors are displacement, velocity and force, etc.