Case Based Questions: Measurement of Length and Motion - 2 | Science for Class 6 PDF Download

Case Study 1: Measuring the Classroom Table with Handspans

Deepa and her friends—Anish, Hardeep, Padma, and Tasneem—decide to measure the length of their classroom table using their handspans. Anish measures slightly more than 13 handspans, Padma measures 13, Tasneem slightly less than 13, Deepa between 13 and 14, and Hardeep 14. They realize the measurements differ because their handspans vary in size. They discuss why body parts like handspans, feet, or fingers lead to inconsistent results and conclude that a standard unit is needed for accurate measurement.

Q1: Why did the number of handspans vary among the friends when measuring the same table?

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Ans: The number of handspans varied because the sizes of their handspans are different from person to person.

Q2: What is a 'unit' in measurement, as explained in the context of their handspan activity?

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Ans: A unit in measurement is the standard used to express the quantity, such as handspan in this case, where the length is expressed as a number followed by the unit (e.g., 13 handspans).

Q3: Suggest one reason why measurements using body parts like handspans differ from person to person.

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Ans: Measurements differ because body parts like handspans, feet, fists, or fingers vary in size from one person to another.

Q4: If they use a metre scale instead, would their results still differ? Explain.

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Ans: No, their results would not differ if they use a metre scale, because it is a standard unit of measurement that remains consistent regardless of who uses it.

Case Study 2: Evolution of Standard Units in Measurement

In ancient India, units like angula (finger width), dhanusa, and yojana were used for measuring artefacts, architecture, and town planning. These are still used by some traditional craftspeople like carpenters and tailors. Over time, different systems of units evolved worldwide, causing confusion during travel. Countries adopted the International System of Units (SI units), where the metre (m) is the standard unit for length. One metre is divided into 100 centimetres (cm), and 1 cm into 10 millimetres (mm). For larger distances, 1 kilometre (km) equals 1000 metres.

Q1: Name two ancient Indian units mentioned for measuring length and give an example of their use.

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Ans: Two ancient Indian units are angula (finger width), used by tailors and carpenters, and yojana, used in town planning.

Q2: Why did confusion arise from different unit systems, and how was it resolved?

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Ans: Confusion arose because people traveling from one place to another encountered different unit systems, leading to inconsistencies; it was resolved by adopting the International System of Units (SI units) as a global standard.

Q3: Convert 1 metre into centimetres and millimetres using the relationships provided.

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Ans: 1 metre = 100 centimetres; 1 metre = 1000 millimetres (since 1 cm = 10 mm, so 100 cm = 1000 mm).

Q4: Would it be convenient to measure the thickness of a book page in kilometres? Explain why or why not.

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Ans: No, it would not be convenient because kilometres are used for larger lengths, and the thickness of a book page is very small, making the measurement impractical (e.g., it would be a tiny fraction like 0.0001 km).

Case Study 3: Correct Measurement Techniques

While measuring the length of a pencil, Deepa ensures the scale is placed in contact with the pencil along its length. She positions her eye directly above the tip of the pencil for accurate reading. If the scale's zero mark is broken, she uses the 1.0 cm mark and subtracts it from the final reading (e.g., 10.4 cm - 1.0 cm = 9.4 cm). For curved objects like the girth of a tree, a flexible measuring tape is used. Visually challenged students use scales with raised markings that can be felt by touch.

Q1: Describe the correct way to place the scale when measuring an object's length.

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Ans: The scale should be placed in contact with the object along its length, ensuring it is straight and aligned properly without gaps.

Q2: Why is the eye position important while reading a scale, and what is the correct position?

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Ans: Eye position is important to avoid parallax error and ensure accurate reading; the correct position is directly above the point being measured (e.g., above the tip of the pencil).

Q3: How can length be measured if the ends of the scale are broken? Give an example.

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Ans: Use any other full mark on the scale (e.g., 1.0 cm), measure from there, and subtract that mark's value from the final reading; for example, if one end reads 1.0 cm and the other 10.4 cm, the length is 10.4 cm - 1.0 cm = 9.4 cm.

Q4:Suggest a tool for measuring the length of a curved line and explain the method.

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Ans: A flexible measuring tape or a thread; place it along the curve, mark the length, then straighten it and measure using a metre scale.

Case Study 4: Describing Positions with Reference Points

Deepa and her friends discuss the distance to a nearby garden for an educational visit. From their houses, Tasneem and Padma think the garden is closer than the school, Deepa and Anish think the school is closer, and Hardeep thinks both are almost equal. When they use the bus stand as a common reference point, their observations align. Later, while drawing lines for a Kabaddi court, they choose a reference point on the ground to measure distances accurately. Padma, traveling by bus to Delhi, reads kilometre stones like 'Delhi 70 km' and 'Delhi 60 km', using Delhi as the reference point to track her position.

Q1: Why did the friends' initial observations about distances to the school and garden differ?

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Ans: Their observations differed because they were measuring distances from their own houses, which are at different locations, rather than from a common reference point.

Q2: What is a reference point, and how does it help in describing position?

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Ans: A reference point is a fixed object or point used to state distance; it helps by providing a consistent basis for comparison, ensuring observations are the same regardless of the observer's location.

Q3: In Padma's bus journey, what does the kilometre stone 'Delhi 70 km' indicate about her position?

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Ans: It indicates that Padma's position is 70 km away from Delhi, with Delhi as the reference point.

Q4: Explain how Padma's position changes with respect to the reference point (Delhi) during her journey.

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Ans: As the bus moves closer to Delhi, the distance on the kilometre stones decreases (e.g., from 70 km to 60 km), showing that her position is changing with time relative to the reference point.

Case Study 5: Observing and Classifying Types of Motion

In a children's park, Deepa observes various motions: a swing moving to and fro (oscillatory), a merry-go-round spinning in a circle (circular), and children sliding down a straight slide (linear). An eraser dropped from a height falls in a straight line (linear motion), while whirling an eraser tied to a thread shows circular motion. A metal strip pressed and released vibrates up and down (oscillatory). Circular and oscillatory motions are periodic as they repeat after fixed intervals. Deepa notes that an object is in motion if its position changes with time relative to a reference point, like passengers in a bus appearing at rest relative to the bus but in motion relative to outside buildings.

Q1: Classify the motion of a swing and a merry-go-round, and justify your classification.

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Ans: Swing: oscillatory motion, as it moves to and fro about a fixed position; Merry-go-round: circular motion, as it moves along a circular path.

Q2: What is linear motion? Give two examples from the park or activities described.

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Ans: Linear motion is when an object moves along a straight line; examples: an eraser dropping from a height (straight down) and children sliding down a straight slide.

Q3: Explain why circular and oscillatory motions are considered periodic.

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Ans: They are periodic because the object repeats its path after a fixed interval of time—circular motion repeats the circle, and oscillatory motion repeats the to-and-fro movement.

Q4: How does the choice of reference point determine if passengers in a moving bus are at rest or in motion?

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Ans: If the reference point is the bus itself, passengers appear at rest (position doesn't change relative to the bus); if it's an outside building, they are in motion (position changes with time relative to the building).