Position, Distance and Displacement
- Concept of position, Concept of distance and displacement
- Distinction between distance and displacement.
Sub-Topic 1: CONCEPT OF POSITION
The position of an object is its location in space. It is usually expressed in relation to a reference point. To locate an object in space, a co-ordinate system is needed. It is usually a mathematical construct with co-ordinates.
A coordinate system could be two-dimensional as in P(x,y) or three dimensional as in P(x,y,z).
Read : MEASUREMENT IN PHYSICS
CONCEPT OF DISTANCE & MEASUREMENT
Distance can be define as a physical measurement of length between two points. It does not take into consideration the direction between the two points it measures; hence, it is a scalar quantity. This therefore means that distance has only magnitude but no direction. E.g, 10km.
Distance could be measured using instruments like measuring tape, ruler, venier calliper, micrometer screw gauge, etc.
CONCEPT OF DISPLACEMENT
Displacement is defined as the distance travelled or moved in a specific direction. It takes into consideration the direction between the different points it seeks to measure; hence, displacement is a vector quantity. Thus, it has both magnitude and direction. E.g, 10km due east. The ‘10km’ is the magnitude (or value), while ‘due east’ is the direction.
Both distance and displacement have the same S.I. unit, metre (m). They could also be expressed in kilometre (km), miles, etc.
NB: Route AC is displacement
Route ABC is distance
- Define distance.
- What is displacement?
- State the SI unit of distance.
Read: Linear Motion
Sub-Topic 2: Distinction between distance and displacement
We need to understand the concepts of distance and displacement. Distance is the gap between two points with no regard to direction. On the other hand, displacement is distance covered in a particular direction. Therefore distance is a scalar quantity while displacement is a vector quantity. The only similarity between distance and displacement is that they have the same unit. Let us consider a girl who walked and covered a distance of 20m between two points A and B as shown in fig 1 and fig 2 below
|A Fig. 1 B
| Fig. 2
A 20m B
The two activities of the girl are not exactly the same. In both figs. 1 and 2, she covered a distance of 20m. If we are only interested in the distance covered, we can conclude that she did the same thing in fig. 1 and 2 i.e she covered the same distance (20m). If we are interested in both distance and direction, then her displacement in fig. 1 and 2 are not the same. In fig.1 she covered a distance of 20m due east while in fig.2, she covered a distance of 20m due west. From these, we see that distance is a scalar quantity because it has magnitude only while displacement is a vector quantity because it has both magnitude and direction.