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💬 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?
Reference Point
Reference
a point everyone agrees to measure from
Point
one fixed spot, called the origin O
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Reference point = a fixed point (origin O) from which every position is measured.
How we describe the position of an object
Step 1 — Fix the origin
choose a reference point O on the line
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Step 2 — Measure the distance
how far the object is from O
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Step 3 — Note the direction
+ on one side of O, − on the other
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Position = distance from the reference point + direction (+ or −)
- 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’?
Is the object at rest or in motion?
An object
Judged from a fixed point
1. At rest
Position does not change
Example:
A parked car
2. In motion
Position changes with time
Example:
A moving train
- 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.
NCERT Question 11 — A student said, “The Earth
A student said, “The Earth moves around the Sun”. In this context, discuss whether an object kept on the Earth can be considered to be at rest.
