4.1.1 Describing position

💬 Where exactly?

How do we say where an object is — and how do we decide whether it is moving or at rest?

How do we describe the position of an object?

First fix a point as the reference point (origin O).
The position of an object at any instant is its distance and direction from the reference point.
For motion in a straight line, there are only two directions — forward and backward — shown by + and − signs. Positions to the right of O are usually positive, to the left negative.

When is an object ‘in motion’ and when ‘at rest’?

If the position of the object with respect to the reference point changes with time, it is said to be in motion .
If its position with respect to the reference point does not change with time, it is at rest .
So rest and motion are always judged relative to a chosen reference point .

Important Definitions

Reference point — a fixed point chosen to describe the position of an object (taken as the origin O).
Position — the distance and direction of an object with respect to the reference point at a given instant of time.
In motion — an object is in motion if its position with respect to the reference point changes with time.
At rest — an object is at rest if its position with respect to the reference point does not change with time.

If the Earth moves around the Sun, can an object kept on the Earth be considered at rest?

Class 6