Displacement

Displacement compared with distance along a path

1. (a) Displacement is a vector quantity which expresses the length and direction of a straight line from one place to another (as opposed to the scalar quantity distance which expresses only the length). The SI unit for either distance or displacement is the metre.

2. (b) Displacement implies movement and is a vector quantity which expesses the distance (in a straight line) between the starting and finishing points.

The two meanings are not the same, as the table shows:

	(a)	(b)
Where are you?	displacement	Position
Where are you relative to the starting point?	Change of displacement	displacement (= change in position)
Velocity	Rate of change of displacement	Rate of displacement

3. Displacement can be related to the measurement of a volume. The displacement of a ship is its weight, which is directly related (via Archimedes' Principle) to the amount of water it displaces when it floats.

The engine displacement of an Otto or Diesel engine is the total amount of air it can draw in during one complete engine cycle; in a piston engine, this can be calculated by multiplying the number of cylinders by the area of a piston by the length of the stroke.

A variation of this third meaning of displacement is a measure of shipping capacity. A ship's size (volume) is reported as the number of long tons (or metric tonnes) of water it displaces. See dead weight tonnage.

4. In psychology, the term displacement refers to an unconscious defence mechanism, whereby the mind redirects emotion from a 'dangerous' object to a 'safe' object. An example would be punching a cushion when angry at your partner.

5. In fencing, a displacement is a movement that avoids or dodges an attack.