NCERT Book Class 6 Science Measurement of Length and Motion PDF Download

Chapter 5: Measurement of Length and Motion

Deepa, a curious eleven-year old girl, lives in a town in the state of Haryana. The new school year has started. Deepa needs a new uniform since she has grown taller. Her mother takes her to a cloth shop. She asks for a two-metre cloth piece. The shopkeeper measures the cloth using a metal measuring rod.

Then, the tailor takes her measurements using a flexible measuring tape. Her mother instructs the tailor to increase the length of her uniform by char angula (four fingers width).

Are the tape and rod similar to the scale that the elder sister has in her geometry box? What did mother mean by char angula?

Deepa shares her experience with her school friends Anish, Hardeep, Padma, Tasneem and this leads to a discussion amongst them.

5.1 How do we Measure?

Hardeep says, "I have seen my grandmother measuring cloth by the length of her arm."

"Have you ever seen how a farmer measures length to divide his field into beds? He walks and counts the number of his strides," says Padma.

"Oh, not just the length of the strides - sometimes they also use the length of their feet to measure," adds Anish.

Deepa says excitedly, "Measuring length using body parts must be so much fun! Let us also measure something using a body part."

"What should we measure? Okay, let us measure the length of the table in our classroom," says Tasneem.

Padma adds, "And which body part should we use to measure it?"

Deepa says, "Let us use our handspan. I will show you how to use it (Fig. 5.1). I have seen my mother using it. She calls it balisht."

[Figure 5.1: Use of handspan for measuring, See in your textbook]

Hardeep adds, "Okay. Let us also note down our measurements."

Teacher's Note

When you measure the same object using different body parts (like handspans), you will get different numbers because everyone's body parts are different sizes. This is why we need a standard unit that stays the same for everyone - so your measurement and your friend's measurement will always match.

Padma says, "Oh, the number of handspans is different for all of us. So, what can we say about the length of the table?"

"But why should the number be different?" Hardeep asked thoughtfully.

Tasneem says, "I can guess. Our handspans are of different sizes."

Anish gives an idea, "Let us check this."

So, all five of them put their handspans along each other and arrive at the conclusion that the lengths of their handspans are different.

Deepa says thoughtfully, "No wonder people use scales and measuring tapes."

Deepa and her friends compare the length of the table with the length of their handspans. The length of the table is expressed in terms of their handspans. Here, the handspan used for measurement is an example of a unit. And the length is expressed in two parts, a number and a unit. For example, if the length of the table is found to be 13 handspans, then 13 is the number and 'handspan' is the unit selected for the measurement.

However, handspans and other similar units, such as length of hand, foot, fist or fingers, differ from person to person. Thus, there is a need for such a unit for which measurements of the same length made by different people do not differ.

Teacher's Note

Notice in Table 5.1 that the same table gave different handspan measurements: 13, 14, or somewhere in between. The number changed because handspans vary in size, but the unit (handspan) stayed the same. This shows why scientists and engineers use standard units like metres or centimetres - so everyone's measurements are exactly the same.

Key Points

• Measurements are made up of two parts: a number and a unit. For example, 13 handspans means 13 is the number and handspan is the unit.
• Body parts like handspans, feet, and fingers are not reliable for measuring because they differ in size from person to person.
• Standard units are needed so that everyone who measures the same length gets the same result, no matter who they are.

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