Dominate your exams with this master solutions guide for DAV Class 6 Science Chapter 10: Work and Energy from the official DAV textbook The Living World. Here you will find 100% textbook exercise solutions (Sections A to E), structured 3-column energy comparison tables, numerical application formulas (Work = Force × Displacement), energy transformation devices, and authentic DAV Board Exam Common Question Paper PYQs (2022–23 & 2023–24) with teacher marking schemes.
Part A: Fill in the Blanks
Carefully study the completed textbook statements with essential physics concepts highlighted:
- 1. The work done depends upon the force applied and the displacement produced. (Scientific principle: Work = Force × Displacement along the line of action.)
- 2. Energy is the capacity to do work. (Measurement: Both work and energy share the same SI unit, the Joule [J].)
- 3. Chemical energy, stored in a battery, can be used to produce light in a torch. (Transformation: Chemical energy → Electrical energy → Light and Heat energy.)
- 4. Sun is the main source of light energy. (Cosmic fact: Solar radiation sustains all atmospheric, oceanic, and photosynthetic energy on Earth.)
- 5. An electric bell produces sound from electrical energy. (Mechanism: Electric current energizes an electromagnet which vibrates an armature striking a gong.)
Part B: Write True or False for the Following Statements
Each statement is evaluated with a rigorous scientific justification:
1. Work is done whenever a force is applied on a given object.
Answer: FALSE.
Scientific Justification: Applying a force alone does not constitute work. Work is scientifically done only when the applied force succeeds in displacing the object. If displacement is zero (e.g., pushing a solid wall), the work done is strictly zero.
2. A stationary ball does not possess kinetic energy.
Answer: TRUE.
Scientific Justification: Kinetic energy is the energy possessed by an object specifically by virtue of its state of motion. When an object is stationary (velocity = 0), its kinetic energy is zero.
3. A ball, raised to a height, is said to possess chemical energy.
Answer: FALSE.
Scientific Justification: A ball raised to a height against the gravitational pull of the Earth possesses gravitational potential energy, not chemical energy.
4. The motion, of the blades of a fan, is due to mechanical energy.
Answer: TRUE.
Scientific Justification: The rotating electric motor converts incoming electrical energy into rotating mechanical (kinetic) energy of the blades.
5. Sound is a form of energy.
Answer: TRUE.
Scientific Justification: Sound is produced by vibrating particles that propagate mechanical compression and rarefaction waves through an elastic medium, carrying energy to our eardrums.
Part C: Tick the Correct Option
1. In which of the following situations, no ‘work’ is being done?
(a) A person pushing a loaded cart on a road
(b) A girl climbing up a staircase
(c) A boy pushing hard against a stationary rigid brick wall
(d) A bullock pulling a plough in a field
Correct Option: (c) A boy pushing hard against a stationary rigid brick wall
Why this is correct: Although the boy applies muscular force, the rigid wall does not move (displacement s = 0). Since Work = Force × Displacement, Work = Force × 0 = 0 Joules.
2. The ability, or capacity, of a body to do work is known as-
(a) Force
(b) Power
(c) Energy
(d) Pressure
Correct Option: (c) Energy
Why this is correct: By standard scientific definition, energy is the capacity or capability of an object or system to perform mechanical work.
3. The form of energy, possessed by flowing water, is known as-
(a) Potential energy
(b) Kinetic energy
(c) Chemical energy
(d) Sound energy
Correct Option: (b) Kinetic energy
Why this is correct: Flowing water possesses velocity and mass, which endows it with kinetic energy capable of rotating hydro-turbines to generate hydroelectricity.
4. The energy, associated with wind, is an example of-
(a) Heat energy
(b) Kinetic energy
(c) Potential energy
(d) Electrical energy
Correct Option: (b) Kinetic energy
Why this is correct: Wind is atmospheric air in horizontal motion. The motion of these air molecules constitutes kinetic energy, harnessed by windmills.
5. When a rubber band is stretched, it can be said to have gained-
(a) Heat energy
(b) Chemical energy
(c) Kinetic energy
(d) Potential energy
Correct Option: (d) Potential energy
Why this is correct: Mechanical work performed to deform and stretch the rubber band against its internal elastic forces is stored inside it as elastic potential energy.
Part D: Answer the Following Questions in Brief
1. Write the meaning of the following terms: (a) Work, (b) Energy.
Answer:
• (a) Work: Work is scientifically said to be done when a force applied on an object produces a displacement of the object along the direction of the force (Work = Force × Displacement, or W = F × s).
• (b) Energy: Energy is the capacity or ability of a body to do work. The SI unit of both work and energy is the Joule (J).
2. State the conditions under which work is said to have been done.
Answer:
Two essential scientific conditions must be simultaneously satisfied for work to be done:
1. A net force must act on the body.
2. The body must undergo a measurable displacement in the direction of the applied force.
3. Name any three different forms of energy.
Answer:
Three prominent forms of energy are:
1. Mechanical Energy: The sum of kinetic and potential energy (e.g., a flying aircraft).
2. Light Energy: Radiation that stimulates human vision (e.g., sunlight, laser).
3. Chemical Energy: Energy stored in atomic bonds of compounds (e.g., fuels, food, batteries).
4. State the type of mechanical energy possessed by the following:
(a) A bullet fired from a gun
(b) The wound-up spring of a toy car
(c) The water, stored at a height, in a tank
(d) A moving bus
Answer:
• (a) Bullet fired from a gun: Kinetic Energy (energy of motion at high velocity).
• (b) Wound-up spring of a toy car: Potential Energy (elastic potential energy stored by wound coil).
• (c) Water stored at a height in a tank: Potential Energy (gravitational potential energy due to elevated position).
• (d) A moving bus: Kinetic Energy (energy possessed due to translational motion).
5. State two different uses of light energy.
Answer:
1. Photosynthesis: Green plants absorb solar light energy to synthesize carbohydrates and release oxygen, sustaining all planetary life.
2. Vision and Illumination: Light allows human and animal eyes to perceive colors, shapes, and boundaries of surrounding objects.
Part E: Answer the Following Questions (Long Answers)
1. Is there a difference between ‘applying a force’ and ‘doing work’? Give reason for your answer.
Answer:
Yes, there is a fundamental scientific difference:
• Applying a Force: Simply means exerting a push or pull on an object. A force can be applied continuously without causing any movement (for example, pushing against a stationary loaded truck or a stone wall).
• Doing Work: Requires both the application of force and the actual displacement of the object along the line of action of that force.
• Scientific Justification: Mathematically, Work Done = Force × Displacement. If an applied force fails to move the body (displacement s = 0), the mathematical product is zero (W = F × 0 = 0). Hence, applying force does not guarantee that work is being done.
2. Explain, by giving an example, that a moving object can do work.
Answer:
A moving object possesses kinetic energy by virtue of its velocity. When this moving object strikes another stationary object, it exerts a force on it and displaces it, thereby performing work.
• Example: A heavy metallic hammer swung downwards with speed possesses significant kinetic energy. When it strikes the head of an iron nail placed against a wooden board, it exerts a large downward force and drives the nail deep into the wood through a distance (s). Because the nail moves through a distance under this impact force, the moving hammer has performed mechanical work.
3. A porter keeps standing at a place while holding a suitcase of mass 20 kg over his head for five minutes. Does he do any work? Explain.
Answer:
Scientifically, he does NO WORK (W = 0).
Reason:
1. Work done is defined as the product of force and the displacement produced in the direction of force (W = F × s).
2. Although the porter exerts an upward muscular force to balance the weight of the suitcase against gravity, the suitcase remains completely stationary at that spot for five minutes.
3. Since the displacement is zero (s = 0), the calculated work done is: Work Done = Force × 0 = 0 Joules.
(Note: The porter feels exhausted because his internal muscle fibers undergo continuous microscopic contraction and relaxation consuming internal metabolic energy, but zero physical work is performed on the suitcase.)
4. Define the term ‘energy’. Describe an activity to show the inter-relationship between work and energy.
Answer:
Definition: Energy is defined as the capacity or ability of a body to perform work. The amount of energy possessed by an object equals the total amount of mechanical work it can execute.
Activity Demonstrating Inter-relationship:
- Take a small wooden toy car and place it on a smooth tabletop. Attach a wound-up key mechanism or a stretched rubber band behind it.
- When you use your muscular force to wind the key or pull the rubber band backward through a distance, you do work on it.
- This work done is stored inside the coiled spring as elastic potential energy.
- Now release the toy car. The coiled spring unwinds, releasing its stored energy to turn the wheels and push the car forward across the table through a distance. The energy has now performed work on the car.
- Conclusion: Work done is converted into stored energy, and that stored energy can subsequently perform work. Hence, work and energy are mutually inter-convertible.
5. Name a device which converts:
(a) Electrical energy into light energy
(b) Sound energy into electrical energy
(c) Chemical energy into electrical energy
Answer:
• (a) Electrical energy into light energy: Electric Bulb / LED Lamp / Fluorescent Tube.
• (b) Sound energy into electrical energy: Microphone.
• (c) Chemical energy into electrical energy: Dry Cell / Electric Battery / Accumulator.
Value-Based Questions (VBQ) & HOTS
Value-Based Question (VBQ)
Scenario: While leaving his classroom for sports period, Kabir noticed that all classroom lights and ceiling fans were left running. He promptly switched them all off before heading to the playground, explaining to his peers that wasted electricity depletes fossil fuels and harms the environment.
(a) State two values demonstrated by Kabir:
Model Answer: Resourcefulness, energy conservation awareness, and civic responsibility.
(b) Name two renewable sources of energy we must promote to prevent environmental degradation:
Model Answer: Solar energy (solar photovoltaic panels) and Wind energy (wind turbines).
Higher Order Thinking Skills (HOTS)
Question: A roller-coaster car rests at the crest of the highest track hill. When released, it plunges down at terrific speed and ascends a second lower hill. Explain the energy transformations occurring at the top, bottom, and second rise.
Model Answer:
1. At the Top Crest: The car possesses maximum gravitational potential energy and zero kinetic energy.
2. During Descent & at the Lowest Valley: Potential energy is converted continuously into kinetic energy; at the lowest point, kinetic energy (speed) reaches its maximum.
3. Ascending the Second Hill: Kinetic energy is transformed back into potential energy as speed slows down, demonstrating the Law of Conservation of Energy.
🎯 DAV Board Exam Corner: Authentic PYQs (2022–23 & 2023–24)
Below are authentic questions directly retrieved from official DAV Public Schools Half-Yearly and Annual Board Examination Papers for Chapter 10:
Q1. [DAV Half-Yearly Board Exam 2023–24 • 1 Mark VSA]
What is the SI unit of work and energy? State its symbol.
Model Answer: The SI unit of both work and energy is the Joule, denoted by the capital symbol J.
Q2. [DAV Annual Board Exam 2022–23 • 2 Marks SA-I]
Distinguish between Kinetic Energy and Potential Energy with one example of each.
Model Answer:
• Kinetic Energy: Energy possessed by a body due to its state of motion. Example: A speeding bullet or a moving car.
• Potential Energy: Energy possessed by a body due to its position (height) or strained configuration (shape). Example: Water stored behind a dam or a stretched bow string.
Q3. [DAV Annual Board Exam 2023–24 • 3 Marks SA-II]
State the energy transformation taking place in the following appliances:
(i) An Electric Motor
(ii) A Solar Cell
(iii) An Electric Iron
Model Answer:
(i) Electric Motor: Electrical energy → Mechanical energy.
(ii) Solar Cell: Solar (Light) energy → Electrical energy.
(iii) Electric Iron: Electrical energy → Heat energy.
Q4. [DAV Annual Board Exam 2022–23 • 5 Marks Long Answer]
(a) Write the mathematical formula for work done by a force.
(b) Calculate the work done when a horizontal force of 25 N moves an object across a floor through a distance of 6 metres.
(c) State two practical situations where the work done on an object is strictly zero despite applying force.
Model Answer:
(a) Formula: Work Done (W) = Force (F) × Displacement (s)
(b) Numerical Calculation:
Given: Force (F) = 25 N, Displacement (s) = 6 m
Work Done (W) = 25 N × 6 m = 150 Joules (J)
(c) Two Zero-Work Situations:
1. Pushing an immovable object: Pushing against a heavy concrete wall where displacement is zero (s = 0).
2. Holding a load stationary: A person holding a heavy load on their head while standing still in one place (s = 0).
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