A shopkeeper's metal rod, a tailor's flexible tape, and grandma's 'char angula' — how do we all agree on the same length?
Key idea
How Do We Measure?
People traditionally measured length using body parts
an arm, a foot, a handspan (balisht).
The problem
When Deepa and her friends measured the same table using their own handspans, they all got different numbers — because everyone's handspan is a different size.
Two parts to a measurement
A measurement always has a number and a unit — like “13 handspans.” 13 is the number, handspan is the unit.
Key idea
Standard (SI) Units of Length
As people began to travel and trade between places, the world adopted one common standard: the International System (SI) of Units.
1 kilometre (km) → = 1000 metres (m) → 1 metre = 100 cm → 1 cm = 10 mm
Key idea
The Correct Way to Measure Length
1
Pick the right scale
A 15-cm scale for a pencil, a metre scale or tape for a room, a flexible tape for a tree's girth.
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2
Keep contact
Place the scale in direct contact with the object, along its full length.
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3
Eye level with the mark
Look straight down at the measurement mark — a tilted eye gives a wrong reading (parallax error).
Key idea
Measuring with a Broken Scale
If a scale's zero mark or end is broken, use any other clear mark, then subtract that mark's reading from the reading at the other end.
Length = 10.4 cm − 1.0 cm = 9.4 cmSubtract the starting reading from the ending reading to get the true length.
Key idea
Measuring the Length of a Curved Line
1
Flexible tape
A flexible measuring tape can be laid directly along a curve — like string lights on an arch.
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2
The thread trick
Lay a thread along the curve, mark its length, then straighten the thread and measure it with a metre scale.
Key idea
Describing Position: The Reference Point
Whether "the garden is closer" is true depends on what point the distance is measured from. Comparing distances only makes sense when measured from the same fixed point.
Reference pointA fixed object or point with respect to which distance or position is described.
Kilometre stonesA road sign reading "Delhi 70 km," then later "Delhi 60 km," shows you're getting closer to Delhi — the reference point.
Key idea
Motion and Rest
1
Motion
An object is in motion if its position changes with respect to a reference point, over time.
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2
Rest
An object is at rest if its position does not change with respect to a reference point, over time. A bus passenger is at rest relative to the bus, but in motion relative to a building outside.
Key idea
Types of Motion
1
Linear motion
A falling eraser, a pushed box
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2
Circular motion
A merry-go-round, a whirled string
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3
Oscillatory motion
A swing, a pendulum
Chapter · Key terms to remember
Key Terms
Metre (m)
The SI unit of length.
Centimetre / Millimetre
1/100 and 1/1000 of a metre.
Reference point
A fixed point used to describe position.
Chapter · Key terms to remember
More Key Terms
Motion
Position changes with time relative to a reference point.
Rest
Position stays the same relative to a reference point.
Linear motion
Motion along a straight line.
Chapter · Key terms to remember
More Key Terms
Circular motion
Motion along a circular path.
Oscillatory motion
To-and-fro motion about a fixed position.
Questions for your notebook
Write these down, then discuss
1
Match each length with the unit best suited to measure it: distance Delhi–Lucknow, thickness of a coin, length of an eraser, length of school ground.
2
True or False? (i) A car moving on a straight road shows linear motion. (ii) An object changing position with respect to a reference point over time is in motion. (iii) 1 km = 100 cm.
3
Which of these is NOT a standard unit of measuring length? (i) millimetre (ii) centimetre (iii) kilometre (iv) handspan
Project as-is — students copy the questions, then the class discusses answers together.