Get authentic, comprehensive, and scoring-oriented solutions for DAV Class 6 Science Chapter 6: Measurement and Motion from the official DAV textbook The Living World. Here you will find step-by-step verified answers to all questions in Something To Know (Sections A, B, C, D, E), structured 3-column motion classification tables, metre scale measurement precautions, Value-Based Questions, and authentic DAV Board Examination Questions (PYQs).
📏 Chapter 6 Quick Revision & Key Concepts Summary:
- Need for Measurement: To make accurate scientific decisions and avoid erroneous estimates based on human senses alone. A complete measurement requires both a number (magnitude) and a standard unit.
- SI Units (International System of Units): Length → Metre (m); Mass → Kilogram (kg); Time → Second (s).
- Rest and Motion: An object is in motion if its position changes with respect to a stationary reference frame over time. An object is at rest if its position does not change with time. Rest and motion are relative terms.
- Translatory Motion: An object moves as a whole along a path. Divided into: Rectilinear motion (straight-line path, e.g., car moving on straight highway, apple falling from tree) and Curvilinear motion (curved path, e.g., ball thrown by a cricketer, car turning on a bend).
- Circular vs Rotational Motion: Circular motion: Object orbits in a circle around an external centre (e.g., revolution of Earth around Sun, movement of tip of clock hand). Rotational motion: Object spins around an internal fixed axis (e.g., spinning of Earth on its axis, rotating potter’s wheel, blades of a ceiling fan).
- Periodic vs Oscillatory Motion: Periodic motion: Repeats itself at regular intervals of time (e.g., phases of moon, pendulum swing). Oscillatory motion: To-and-fro motion of an object about its fixed mean position (e.g., swing, bob of a simple pendulum, plucked guitar string). All oscillatory motions are periodic, but all periodic motions are not oscillatory.
Comprehensive 3-Column Classification: Types of Motion
| Type of Motion | Characteristic Motion Path | Two Real-World Examples |
|---|---|---|
| 1. Rectilinear Motion | Motion along a strictly straight line | A car on a straight road, an athlete in a 100m straight race |
| 2. Curvilinear Motion | Motion along a curved path | A javelin thrown into the air, a car navigating a sharp curve |
| 3. Circular Motion | Motion along a circular circumference around a fixed centre | Pedal of a bicycle, motion of the moon around the Earth |
| 4. Rotational Motion | Spinning of a body around an internal axis passing through it | A spinning top, rotation of the Earth on its axis, rotating potter’s wheel |
| 5. Oscillatory Motion | To-and-fro motion about a fixed mean central position | Motion of a playground swing, bob of a simple pendulum |
A. Fill in the blanks.
1. The standard unit of mass is the kilogram.
Explanation: In the International System of Units (SI), mass is universally quantified in kilograms (kg).
2. While sitting in a moving bus, we are in motion with respect to the trees on the roadside.
Explanation: Our position relative to the stationary trees changes continuously with time as the bus travels.
3. A moving car, going on a narrow straight road, is having a (nearly) rectilinear motion.
Explanation: Motion along an uncurving, straight track is classified as rectilinear translatory motion.
4. The motion, of the needle of a sewing machine, is an example of an oscillatory / periodic motion.
Explanation: The needle moves rapidly up and down continuously about a fixed position in regular time intervals.
5. The motion of the earth, around the sun, is circular / periodic / revolution motion.
Explanation: Earth orbits around the Sun in an elliptical/circular path taking precisely 365.25 days (periodic revolution).
B. Write True or False for the following statements.
1. Measurement is needed to make correct decisions.
Answer: True
Explanation: Exact measurement ensures quantitative accuracy in construction, medicine doses, sports timing, and commercial trade.
2. A metre scale can be used for measuring the length of a playground.
Answer: False
Correction: A small one-metre rigid scale is unsuitable and prone to cumulative error; a long measuring tape is used for measuring large playgrounds.
3. The motion of the moon, around the earth, is a rectilinear motion.
Answer: False
Correction: The moon moves in a curved orbital trajectory around Earth, so it executes circular and periodic motion.
4. The motion, of the pedal of a bicycle, is a circular motion.
Answer: True
Explanation: When pedalling, each pedal traverses a circular path at a constant radial distance around the pedal axle.
5. The motion, of a giant wheel, is a rotatory motion.
Answer: True
Explanation: The entire giant wheel spins on a fixed central horizontal axle.
6. When we draw water from a well, we are pushing at the rope.
Answer: False
Correction: When drawing water, we exert a tension force by pulling the rope over the pulley.
C. Tick the correct option.
1. The standard unit of length is the—
(a) centimetre
(b) metre
(c) kilometre
(d) millimetre
Answer: (b) metre
Reason: In the SI metric system, metre (m) is the foundational standard unit of length.
2. The standard unit of time is the—
(a) hour
(b) minute
(c) second
(d) day
Answer: (c) second
Reason: The second (s) is the international SI base unit of time.
3. The motion, of the tip of a pen, while writing, is an—
(a) rectilinear motion
(b) circular motion
(c) curvilinear motion
(d) oscillatory motion
Answer: (c) curvilinear motion
Reason: While forming letters and words, the tip of the pen follows irregular curved paths across the paper.
4. Which of the following four ‘motions’ is different from the other three?
(a) the motion of a spinning top
(b) the motion of a potter’s wheel
(c) the motion of a blade of a rotating ceiling fan
(d) the motion of a falling stone
Answer: (d) the motion of a falling stone
Reason: A falling stone exhibits linear/rectilinear translatory motion, whereas the other three execute rotational motion around an axis.
5. The motion of a swinging pendulum is a/an—
(a) rotatory motion
(b) circular motion
(c) rectilinear motion
(d) oscillatory motion
Answer: (d) oscillatory motion
Reason: The pendulum bob swings back and forth along an arc about its equilibrium central position.
D. Answer the following questions in brief.
1. How does measurement help us in our day-to-day life?
Answer: Measurement is essential in day-to-day life because:
1. It provides exact quantitative values for buying and selling goods (vegetables, milk, cloth).
2. It allows accurate administration of medical dosages by doctors and pharmacists.
3. It enables architects and carpenters to build houses, furniture, and roads with precision.
4. It ensures accurate timekeeping in scheduling school, train, and flight timetables.
2. Which type of watch is used to measure time intervals in a 200-metre race?
Answer: A stopwatch (or digital electronic stop-timer) is used because it can measure tiny fractions of a second (up to one-hundredth of a second) and can be started and stopped instantaneously.
3. When do we consider an object to be in motion?
Answer: An object is considered to be in motion when its position changes continuously with respect to its stationary surroundings (reference point) as time passes.
4. Name three different types of motion and give one example of each.
Answer:
1. Rectilinear motion: A train moving smoothly along a straight rail track.
2. Circular motion: The motion of the tip of the second hand of a wristwatch.
3. Oscillatory motion: The to-and-fro motion of a child on a playground swing.
E. Answer the following questions.
1. Why does a measurement need both a ‘number’ and a ‘unit’ for its complete specification?
Answer: A measurement is incomplete and ambiguous without both a number and a unit:
1. Role of the Number (Magnitude): The number conveys how many times the standard quantity is contained in the measured entity.
2. Role of the Unit: The unit defines the standard benchmark against which the comparison is being performed.
Example: If a student says the length of a desk is “5”, it is meaningless because we do not know whether it is 5 centimetres, 5 inches, 5 spans, or 5 metres. Stating “5 metres” specifies the measurement completely and unambiguously.
2. Why is a foot step, or a hand span, not used as a standard unit of length?
Answer: Non-standard body units like footsteps, cubits, or hand spans cannot serve as standard units because:
1. Individual Variability: Anatomical dimensions vary widely from person to person. An adult’s hand span is much larger than a child’s hand span.
2. Lack of Universal Acceptance: Measuring the same piece of cloth will produce different readings when measured by different individuals, leading to disputes.
3. Invariance: A scientific standard unit must remain constant across all locations and times, a property body units completely lack.
3. Radha was performing an experiment in the Science Laboratory. The metre scale, that she was using, had its zero mark broken.
(a) She measures the length of a string by keeping its one end at the 3.0 cm mark. If the other end reads 28.5 cm, what is the actual length of the string?
(b) What precaution must Radha take while using such a scale?
Answer:
• (a) Calculation:
Initial Reading = 3.0 cm
Final Reading = 28.5 cm
Actual Length of String = Final Reading − Initial Reading = 28.5 cm − 3.0 cm = 25.5 cm
• (b) Precaution: When the zero mark is damaged or broken, one must avoid using the zero end. Instead, align one end of the object with any clear, complete full-centimetre graduation mark (e.g., 1 cm or 3 cm) and strictly subtract this initial reading from the final mark reading to avoid error.
4. Explain the difference between ‘rest’ and ‘motion’. Give two examples of each.
Answer:
• Rest: A body is said to be at rest if it does not change its position with respect to its stationary surroundings with time.
Examples: (1) A textbook resting quietly on a table; (2) An electric pole fixed on the pavement.
• Motion: A body is said to be in motion if it changes its position with respect to its stationary surroundings with time.
Examples: (1) A speeding car on a highway; (2) A bird flying in the sky.
5. Distinguish between: (a) Rectilinear and Curvilinear motion (b) Rotation and Revolution (c) Periodic and Non-periodic motion.
Answer:
• (a) Rectilinear vs Curvilinear Motion:
– Rectilinear: Motion in which an object travels along a straight line (e.g., an apple falling freely from a tree).
– Curvilinear: Motion in which an object travels along a curved path (e.g., a ball thrown in a curved arc by a basketball player).
• (b) Rotation vs Revolution:
– Rotation: Spinning of a body around an internal axis passing through its own body (e.g., spinning of Earth on its axis once in 24 hours, potter’s wheel).
– Revolution: Movement of a body in an orbit around an external centre or body (e.g., revolution of Earth around the Sun in 365 days).
• (c) Periodic vs Non-periodic Motion:
– Periodic: Motion that repeats itself at regular, fixed intervals of time (e.g., oscillation of pendulum, heartbeat).
– Non-periodic: Motion that does not repeat itself at regular intervals of time (e.g., a footballer sprinting across a pitch, an earthquake).
Precautions to be Followed While Using a Metre Scale (Standard 5-Marker)
To obtain accurate measurement of length using a standard metre scale without experimental error, five essential precautions must be observed:
- Placement of Scale: The metre scale must be placed strictly parallel and in close contact along the entire length of the object being measured without any gap or tilt.
- Avoiding Worn or Broken Zero Ends: If the zero mark is worn out or damaged, avoid starting from the zero mark. Place the object against a distinct, undamaged mark (such as 1.0 cm) and subtract that reading from the final reading.
- Correct Position of the Eye (Avoiding Parallax Error): The observer’s eye must be positioned vertically right above the point where the reading is being taken. Viewing from an angle to the left or right introduces parallax error and causes false readings.
- Using Straight Edge: The measuring edge of the scale must be straight and flat against the surface; worn-out wooden scales with chipped bevels should not be used.
- Measuring Curved Lines: A rigid metre scale cannot measure curved lines directly. First, trace the curved line using a flexible cotton thread or divider, and then measure the stretched thread along the metre scale.
Value-Based Questions (VBQ) & HOTS (Higher Order Thinking Skills)
💡 Value-Based Question (VBQ):
Scenario: While purchasing ribbon at a local fair, a vendor measured the length using the cubit (elbow to fingertip length). Suman politely requested the vendor to use a certified steel metre rod instead, explaining that arm lengths differ.
Values Displayed:
1. Fairness & Scientific Awareness: Suman stood up for standardized commercial ethics and scientific accuracy.
2. Polite Communication: She resolved the situation courteously by educating the vendor on standardized units without conflict.
🧠 Higher Order Thinking Skill (HOTS):
Question: When a bicycle moves on a straight road, what different types of motion are executed simultaneously by its various parts?
Multiple Simultaneous Motions:
1. Rectilinear Motion: The bicycle as a whole, including the rider, moves forward in a straight line along the road.
2. Rotational Motion: The wheels spin around their central fixed axle.
3. Circular Motion: A point marked on the rim or the valve pin of the tyre executes circular motion around the hub.
4. Periodic / Circular Motion: The cyclist’s feet and pedals move in circles around the central gear crank at regular cadence.
🎯 DAV Examination Corner: Authentic Board Exam Questions (PYQs 2022–23 & 2023–24)
Below are authentic questions directly retrieved from the DAV Public Schools Common Examination (Half-Yearly & Annual Board Papers) covering Chapter 6:
Q1. [DAV Annual Board Exam 2022–23 • 1 Mark VSA]
Give two examples of oscillatory motion.
Model Answer: (1) Motion of the pendulum of a wall clock; (2) Motion of a child on a swing.
Q2. [DAV Annual Board Exam 2022–23 • 2 Marks SA-I]
Differentiate between rectilinear and curvilinear motion.
Model Answer:
• Rectilinear Motion: When an object moves along a straight line path (e.g., a bullet fired from a gun, a car on a straight track).
• Curvilinear Motion: When an object moves along a curved path (e.g., a javelin thrown by an athlete, a car turning at a roundabout).
Q3. [DAV Annual Board Exam 2022–23 • 2 Marks SA-I]
Why is foot step or a hand span not used as a standard unit of length?
Model Answer: The length of a foot step or hand span differs significantly from person to person. Because a standard scientific unit must remain constant and invariable for everyone everywhere, human body spans cannot be used as standard units.
Q4. [DAV Annual Board Exam 2022–23 • 5 Marks Long Answer]
Write five precautions to be followed when a metre scale is being used for measuring a given length.
Model Answer:
1. Place the metre scale strictly parallel and in direct contact with the object being measured.
2. Avoid using worn-out or chipped zero marks; start from a full mark (e.g., 1.0 cm) and subtract that mark from the final reading.
3. Keep the eye positioned vertically right in front of the mark being read to prevent parallax error.
4. Ensure the scale does not bend or tilt during the reading.
5. For measuring curved lengths, use a thread or divider first, then measure the stretched thread on the scale.
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