Measurement of Length and Motion | Science Grade 6 CBSE

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Six ideas about measuring and moving

From why we need a standard metre to the different ways things move. Tap each term to see what it means and where it shows up.

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The three big ideas

Long ago people measured with body parts – a handspan, a cubit (elbow to fingertip), a footstep, a pace. The problem: these differ from person to person, so a “three-handspan” cloth was never the same length twice.
We need standard units that are the same everywhere and for everyone. The agreed system is the SI (International System of Units).
The SI unit of length is the metre (m). Useful relations: 1 m = 100 cm, 1 cm = 10 mm, and 1 km = 1000 m.
Choose a sensible unit: km for road distances, m for a room, cm or mm for a pencil.

Place the scale in contact with the object, along its full length.
If the zero end is worn, start from another clear mark and subtract the two readings.
Keep your eye directly above the mark you are reading. Looking from an angle gives a wrong reading – this is called a parallax error.
For a curved line (like a winding path on a map), lay a thread along the curve, mark its ends, then straighten the thread against a scale.

Worked example. A pencil’s left end is at 1.0 cm and its right end at 13.0 cm on a ruler with a broken zero. How long is it?

Length = right reading − left reading = 13.0 − 1.0 = 12.0 cm.

Common mistake: reading the scale from an angle (parallax), or measuring from the very edge of the ruler instead of the 0 mark.

An object is in motion if its position changes with time compared to a fixed reference; otherwise it is at rest. Motion and rest are relative – a passenger is at rest with respect to the bus but in motion with respect to the road.
Rectilinear (straight-line) motion – a car on a straight road, a falling stone, a marching parade.
Circular motion – the tip of a clock’s hand, a stone whirled on a string, a merry-go-round.
Rotational motion – a spinning top, a ceiling fan, the Earth turning on its axis.
Periodic (to-and-fro) motion – a swing, a pendulum, a vibrating guitar string.
A moving object can show more than one type at once – a rolling ball moves forward (rectilinear) while spinning (rotational).

Where you’ll meet it

Measurement at work

The tailor’s tape

A tailor measures your sleeve and waist in centimetres with a flexible tape that bends around curves – the same thread-along-a-curve idea, so the stitched clothes fit exactly.

The odometer in a vehicle

The odometer on a scooter or bus counts how many kilometres the wheels have rolled. Standard units mean a “200 km” trip means the very same distance to everyone on the road.

Sports and records

A long-jump or a 100-metre sprint is judged by careful measurement in metres and seconds. Without standard units, no record from one ground could ever be compared with another.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and case studies, testing whether you can use the ideas, not just recall them.

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