How do we Measure?
Class 6 Science · Chapter 5: Measurement of Length and Motion · NCERT Curiosity section 5.1
Learn
Gauri and her little brother measure the same table with their handspans. Gauri gets 8 handspans; her brother gets 11! Who is right? Both counted correctly, but their hands are different sizes.
Measuring means comparing an unknown quantity with a known fixed quantity called a unit. A measurement has two parts: a number and a unit. "8 handspans": 8 is the number, handspan is the unit.
measurement = number + unit
People have long used body parts as units: the handspan, the cubit (elbow to fingertip), the footstep, the finger width (in old India, the angula). They are handy, but they vary from person to person, so results do not agree.
Even in the ancient Harappan cities, people used marked scales for building, because traders and builders needed lengths that everyone agreed on. We need standard units: the same everywhere, for everyone.
Explore: measure with a ruler instead of a hand.
- Tap the mark at each end of the pencil.
Remember
- Measuring compares a quantity with a unit; a measurement is a number plus a unit.
- Body-part units vary from person to person, so we need standard units.
Quick check
Why do two children get different numbers of handspans for the same table?
- Their hands are different sizes
- The table changes size
- One of them cannot count
- Handspans are standard units
Answer
Their hands are different sizes. Body-part units vary, which is why we need standard units.
Think about it
Measure your classroom door with your handspan and ask a friend to do the same. Why might your answers differ, even if you both count carefully?
One answer
Your handspans are not the same length, and each handspan may also be stretched a little differently each time. A standard unit like the centimetre removes this problem.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.