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 expresses the distance (in a straight line) between the starting and finishing points.
The two meanings are not the same, as the table shows:
Distance and displacement are two quantities which may seem to mean the same thing, yet they have distinctly different meanings and definitions.
Displacement is a vector quantity which refers to "how far out of place an object is"; it is the object's change in position.
To understand the distinction between distance and displacement, you must know their definitions and you must also know that a vector quantity such as displacement is direction-aware whereas a scalar quantity such as distance is ignorant of direction.
In terms of distance, quite far (the circumference of the earth's orbit is nearly one trillion meters), but in terms of displacement, not far at all (zero, actually).
Distance and displacement are said to be isotropic, that is, they remain unchanged even if the coordinate system undergoes translation or rotation.
The distance from the sun to Mars is 1.5 AU; from the sun to Jupiter, 5.2 AU; and from the sun to Pluto, 40 AU.
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