Class 6 Science Chapter 5: A Complete Guide to the Measurement of Length and Motion

Introduction: The Tale of a New Uniform 📏

The new school year has started, and for eleven-year-old Deepa, that means it's time for a new uniform. She has grown taller over the last year, and her old one is now too short. Her mother takes her to the local cloth shop, where a simple request for a "two-metre cloth piece" starts a fascinating journey into the world of measurement.

At the shop, Deepa watches as the shopkeeper uses a long, rigid metal rod to measure the cloth. Later, at the tailor's, she sees a completely different tool: a flexible measuring tape that wraps easily around her for her measurements. Then, she hears her mother give a curious instruction to the tailor: "increase the length of her uniform by char angula (four fingers width)." A metal rod, a flexible tape, and four fingers—all used to measure length!

This experience leaves Deepa with many questions. Are these all correct ways to measure? Why use different tools for different jobs? And how can everyone agree on a length if one person uses a metal rod and another uses their fingers? This chapter will answer these questions and explore the science of how we measure length and describe motion.

What We Will Learn in This Chapter 📖

Here are the key topics we will cover to understand the world of measurement and motion:

• Why we need to measure things in the first place.
• Understanding the difference between non-standard and standard units of measurement.
• The International System of Units (SI Units) that scientists and countries use today.
• The correct techniques for measuring length accurately with a scale.
• A clever method for measuring the length of curved lines.
• How to describe an object's position using a reference point.
• What motion is and its different types: Linear, Circular, and Oscillatory.

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From Handspans to Metres: The Story of Measurement 🖐️

Long before we had rulers and measuring tapes, people used what was easily available: their own bodies. Deepa's friends shared stories of how their elders measured things. Hardeep mentioned seeing his grandmother measure cloth by the length of her arm. “Have you ever seen how a farmer measures length to divide his field into beds?” asks Padma. “He walks and counts the number of his strides.” Anish adds, “Oh, not just the length of the strides—sometimes they also use the length of their feet to measure.” Deepa's mother used her handspan, a measurement she called a balisht.

Curious, Deepa and her friends decided to try this themselves by measuring their classroom table using their handspans. Here are the results they got:

Table 5.1: Measuring the length of the table | Name of the Student | Number of Handspans | | :------------------ | :-------------------- | | Anish | Slightly more than 13 | | Padma | 13 | | Tasneem | Slightly less than 13 | | Deepa | Between 13 and 14 | | Hardeep | 14 |

They all measured the same table, but as Padma pointed out, “Oh, the number of handspans is different for all of us!” “But why should the number be different?” Hardeep asked thoughtfully. Tasneem had a guess: “I can guess. Our handspans are of different sizes.” They were right! This experiment teaches us a crucial lesson: a measurement must have two parts—a number and a unit (like '13' and 'handspans'). But if the unit changes from person to person, the measurement is unreliable. This is why we need standard units.

Getting on the Same Page: Standard Units 🌍

To avoid the confusion caused by handspans and strides, countries around the world came together and agreed on a uniform system of measurement. This system is called the International System of Units (SI units).

For length, the SI unit is the metre (m).

To measure lengths that are much bigger or smaller than a metre, we use other units that are based on the metre. But would it be convenient to use the unit metre to measure larger lengths, such as the length of a railway track between two cities, or to measure smaller lengths, such as the thickness of a page of a book? For this, we have other units:

• Centimetre (cm): For smaller measurements, one metre is divided into 100 equal parts. Each part is called a centimetre.
• Millimetre (mm): For even smaller measurements, one centimetre is divided into 10 equal parts. Each part is a millimetre. A millimetre is often the smallest marking you will see on a typical 15-cm school scale.
• Kilometre (km): For measuring large distances, like the distance between two cities, we use the kilometre. One kilometre is equal to 1000 metres.

Interestingly, India has a rich history of its own measurement systems. Ancient units like angula (finger width), dhanusa, and yojana were used in architecture and town planning for centuries. The angula is still used by traditional craftspeople like carpenters and tailors.

Measuring Like a Pro: The Right Way to Use a Scale ✅

Using a standard unit is the first step, but using your measuring tool correctly is just as important for getting an accurate result. Here are four key points to remember:

1. Choose the Right Tool: You must use a scale that is appropriate for the job. You would use a 15-cm scale to measure a pencil, a metre scale for the length of a room, and a flexible measuring tape for curved surfaces like the girth of a tree or a person's chest.
2. Place the Scale Correctly: The scale must be placed in direct contact with the object, right along the length you want to measure. It should not be tilted or at an angle.
3. Position Your Eye Correctly: To avoid errors, your eye should be positioned directly above the mark you are reading (Position 'B' in Fig. 5.5). Looking from the side can make the reading appear shifted, leading to a mistake.
4. Measure with a Broken Scale: What if the zero mark on your scale is broken or unclear? No problem! You can start your measurement from any other clear, full mark (like the 1.0 cm mark). Just remember to subtract this starting value from your final reading. For example, if you start at 1.0 cm and the object ends at 10.4 cm, the actual length is 10.4 cm – 1.0 cm = 9.4 cm.

What About Wiggly Lines? Measuring Curves ➰

You can't use a rigid, straight scale to measure a curved line, like the length of decorative string lights needed for a verandah arch. So, how is it done? The correct method uses a thread:

• First, carefully place a thread along the exact path of the curved line.
• Next, straighten the thread out without stretching it.
• Finally, measure the length of the now-straight thread using a standard scale.

This simple technique allows you to accurately measure the length of any curve.

Where Are You? The Importance of a Reference Point 📍

When Deepa and her friends discussed whether the garden was closer or farther than their school, they all had different answers. This was because each of them was thinking about the distance from their own house. If they had all measured the distance from a single, common point—like the bus stand—their observations would have been the same.

This fixed object or point from which distance is measured is called a reference point.

Another great example is when Padma was travelling on a bus and saw kilometre stones reading 'Delhi 70 km'. In this situation, the city of Delhi is the reference point. All the distances are being measured from that single, fixed location. The same idea applies when the friends need to draw a Kabaddi court for sports day. To draw the lines correctly, Deepa suggests they first decide on a point on the ground to measure from. That starting point becomes their reference point for the whole court.

Are We Moving? Understanding Motion and Rest 🚶‍♂️🌳

How do we decide if something is moving? It's all about the reference point!

• Motion is the state where an object's position changes with time with respect to a reference point.
• Rest is the state where an object's position does not change with time with respect to a reference point.

This means motion is relative. Think about a passenger on a moving bus. With respect to the bus itself (or another passenger), the person is at rest. But with respect to a tree on the side of the road, the person is in motion because their position relative to the tree is constantly changing.

Different Ways to Move: Types of Motion 🔄

Objects can move in different ways. Here are three main types of motion we see all around us.

Linear Motion

• Definition: Motion along a straight line.
• Examples: An object dropped from a height, a car driving on a straight road, or soldiers marching in a parade.

Circular Motion

• Definition: Motion along a circular path.
• Examples: An eraser being whirled on the end of a thread, a child on a merry-go-round, or the blades of a moving ceiling fan.

Oscillatory Motion

• Definition: Motion that repeats back and forth (to and fro) about a fixed position.
• Examples: A child on a swing or a thin metal strip that is fixed at one end and made to vibrate.

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Key Takeaways and Formulas ✨

As you work with measurements, it's important to write them correctly. Here are the official rules:

• Unit names (like metre, kilometre) always start with a lowercase letter, unless they are at the beginning of a sentence.
• Symbols for units (m, cm, km) are also written in lowercase.
• Symbols are never followed by 's' to make them plural. It is '10 km', not '10 kms'.
• Do not put a full stop after a unit symbol (unless it's at the very end of a sentence).
• Always leave a space between the number and the unit symbol (for example, write '15 cm', not '15cm').

Unit Conversions:

• 1 kilometre (km) = 1000 metres (m)
• 1 metre (m) = 100 centimetres (cm)
• 1 centimetre (cm) = 10 millimetres (mm)

Did You Know? 🤔

• In earlier days, units like the inch and foot were used to measure length. Even today, these are sometimes used, and we know that 1 inch is equal to 2.54 cm!
• Archaeological excavations from the ancient Harappan Civilisation have found objects with ruled markings on them, which could be some of the earliest scales ever made.
• Visually challenged students use special scales that have raised markings. They can feel these markings with their fingers to measure lengths accurately.

Exam Corner: Frequently Asked Questions (FAQs) 📝

1. Why is a 'handspan' not a standard unit of measurement? A handspan is not a standard unit because its length is different for every person. This leads to measurements that are inconsistent and unreliable.
2. Which tool is best for measuring the girth (circumference) of a tree? A flexible measuring tape is the most suitable tool. A rigid scale cannot bend to measure a curved surface, but a flexible tape can wrap around the tree for an accurate measurement.
3. A book measures 15.5 cm. A student started measuring from the 2.0 cm mark on the scale. What was the final reading on the scale? To find the final reading, you add the starting point to the actual length of the object. So, the calculation is: 15.5 cm (length) + 2.0 cm (start point) = 17.5 cm. The final reading on the scale would be 17.5 cm.
4. What is the difference between circular and oscillatory motion? The main difference is the path of movement. Circular motion is movement along a circular path, like a merry-go-round. Oscillatory motion is a back-and-forth movement around a fixed position, like a child on a swing.
5. A passenger is sitting on a moving bus. Are they at rest or in motion? This depends on the reference point! The passenger is at rest with respect to another passenger on the bus. However, the passenger is in motion with respect to a tree on the roadside because their position relative to the tree is changing.

Conclusion and Exam Tips

From using our hands to measure a table to using the SI system to measure the distance between cities, this chapter has shown us the importance of accurate and standard measurement. We've also learned to identify the different ways objects move around us—in straight lines, in circles, or back and forth. Understanding these basic concepts is the first step to exploring the wider world of physics!

Exam Tips:

• Memorize the key unit conversions: km to m, m to cm, and cm to mm.
• Practice calculating with a broken scale. Know how to find the length (Final Reading - Start Reading) and how to find the final reading if you know the length (Start Reading + Length).
• Be able to clearly define and give two examples for linear, circular, and oscillatory motion.
• Always remember to write both the number and the correct unit in your answers!