Complete Solutions and Summary of Work and Energy – NCERT Class 9, Science, Chapter 10 – Summary, Questions, Answers, Extra Questions
Detailed summary and explanation of Chapter 10 ‘Work and Energy’ with all question answers, extra questions, and solutions from NCERT Class IX, Science.
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Tags: Work, Energy, Power, Kinetic Energy, Potential Energy, Conservation of Energy, Mechanical Energy, Joule, Watt, Class 9, NCERT, Science, Chapter 10, Answers, Extra Questions
Work and Energy
Chapter 10: Science - Complete Study Guide
Chapter Overview
Work
F × s
Energy
Joule
Kinetic
½mv²
Potential
mgh
What You'll Learn
Work Definition
Force × displacement in direction of force.
Energy Forms
Kinetic (motion) and potential (position).
Conservation Law
Energy transforms but total remains constant.
Power
Rate of doing work, in watts.
Key Highlights
Work requires force and displacement. Energy enables work: kinetic from motion, potential from height. Law of conservation: energy changes form but isn't created/destroyed. Power measures work rate. Units: joule for work/energy, watt for power.
Comprehensive Chapter Summary
1. Introduction to Work and Energy
- Living beings need energy from food for life processes like survival activities.
- Energy required for strenuous activities like playing, running; animals for jumping, fighting, finding food.
- Machines need fuel like petrol/diesel for working, as they require energy similar to living beings.
- Work in daily life differs from science: e.g., studying hard expends energy but may not involve scientific work if no displacement.
- Examples: Pushing immovable rock (no work, no displacement); holding load (no work, no displacement); climbing stairs (work done against gravity).
- Scientific work: Force acts and object displaces in force direction.
- To double content: Energy sources include Sun (solar, wind, fossil fuels derived), nuclei, earth interior, tides; machines convert chemical to mechanical energy.
Activity 10.1: Everyday Work Analysis
Discuss activities like reading, pushing: Identify object, change, who does work. Reveals scientific vs. daily work.
2. Scientific Conception of Work
- Work done if: (i) Force acts on object, (ii) Object displaces.
- Examples: Pushing pebble (work), pulling trolley (work), lifting book (work), bullock cart (work).
- No work if force but no displacement (e.g., stationary object) or displacement but no force (e.g., free fall after push).
- Work by constant force: \( W = F \times s \) (in direction of force).
- Unit: Joule (J) = 1 N × 1 m.
- Positive work: Force and displacement same direction.
- Negative work: Opposite direction (e.g., retarding force).
- To double: Conditions must both satisfy; zero force or zero displacement means zero work. In vectors, work is dot product \( \vec{W} = \vec{F} \cdot \vec{s} = Fs \cos \theta \).
Work Formula
- \( W = F s \cos \theta \)
- θ=0°: Positive, max work.
- θ=90°: Zero work.
- θ=180°: Negative.
Examples from PDF
- 5N force, 2m displacement: 10J.
- 7N force, 8m: 56J.
- Porter lifts 15kg, 1.5m: 225J.
Activity 10.2: Daily Situations
List work scenarios: Force, object, change. Discuss if work done.
Activity 10.3: Force-Displacement
Situations: No displacement with force (zero work); displacement without force (zero work).
Activity 10.4: Positive/Negative Work
Lift object: Your force positive, gravity negative.
3. Energy Concepts
- Energy: Capacity to do work; unit joule (1J = work of 1N over 1m).
- Forms: Mechanical (kinetic + potential), heat, chemical, electrical, light.
- Object with energy exerts force, transfers to another, causing motion/work.
- Sources: Sun (primary), atomic nuclei, earth's interior, tides.
- To double: Energy measured by work capacity; larger unit kJ=1000J. Examples: Hammer drives nail (potential to kinetic), wound toy moves (stored to kinetic), balloon explodes (elastic to sound/heat).
Activity 10.5: Energy Sources
List sources: Discuss Sun-derived (fossil fuels, wind) vs. others (nuclear, geothermal).
4. Kinetic Energy
- Moving objects do work; faster motion = more work (e.g., bullet pierces, wind turns mill).
- Kinetic energy (KE): Due to motion; \( KE = \frac{1}{2} m v^2 \).
- Work to acquire velocity equals KE change.
- Derivation: From \( v^2 - u^2 = 2as \), \( W = F s = m a s = \frac{1}{2} m (v^2 - u^2) \).
- If u=0, \( KE = \frac{1}{2} m v^2 \).
- To double: KE increases with speed squared; examples: falling ball dents deeper from height (more speed), trolley displaces block more with mass (more KE).
Activity 10.6: Falling Ball
Drop from heights: Deeper dents from higher (more KE).
Activity 10.7: Trolley Experiment
Mass on pan moves trolley, hits block: Displacement increases with mass (more KE transferred).
5. Potential Energy
- Stored energy from position/configuration (e.g., stretched rubber, wound spring, raised object).
- Gravitational PE: \( PE = m g h \) (work against gravity).
- Depends on reference level; work by gravity on height difference, path-independent.
- Elastic PE: From shape change (bow, slinky).
- To double: PE gained equals work done; examples: Stretched band regains shape (releases PE), toy car winds store PE in spring, bow launches arrow (PE to KE).
Activity 10.8: Rubber Band
Stretch/release: Acquires/released PE.
Activity 10.9: Slinky
Stretch/compress: Stores PE in deformed state.
Activity 10.10: Toy Car
Wind key: More turns = more PE stored.
Activity 10.11: Lift Object
Higher lift = more PE; falls releasing to KE.
Activity 10.12: Bow and Arrow
Stretch bow: Shape change stores PE, releases to KE for arrow flight.
6. Energy Transformations and Conservation
- Forms interconvertible: e.g., plants (light to chemical), wind (kinetic), fuels (chemical to heat).
- Human activities: Bulb (electrical to light/heat), bicycle (chemical to mechanical).
- Law: Energy conserved; total unchanged, transforms forms.
- Free fall: Initial PE = mgh, converts to KE = ½mv²; total constant (ignores air resistance).
- Mechanical energy: KE + PE = constant.
- To double: Nature examples: Photosynthesis (solar to chemical), water cycle (solar to potential/kinetic), fossil fuels (ancient solar). Human: Gadgets like fan (electrical to kinetic).
Activity 10.13: Nature Conversions
Discuss: Plants food (light to chemical), air movement (solar heat), fuels (solar ancient), water cycle (solar evaporation).
Activity 10.14: Human Gadgets
List: e.g., bulb (electrical to light), car (chemical to kinetic).
Activity 10.15: Free Fall Table
20kg from 4m: PE decreases, KE increases, total 800J constant.
7. Rate of Doing Work: Power
- Power: Work/time or energy transfer rate; \( P = W/t \).
- Unit: Watt (W) = 1 J/s; kW = 1000 W.
- Average power: Total work/total time.
- Examples: Faster climber higher power; stronger engine quicker journey.
- To double: Varies with time; meter readings show day/night consumption for bills.
Activity 10.16: Climbing Rope
Two children climb 8m: Same work, A (15s) > power than B (20s).
Activity 10.17: Electric Meter
Read AM/PM: Tabulate units (kWh), infer usage patterns vs. bill.
Key Concepts and Definitions
Work
\( W = F s \), joule.
Kinetic Energy
\( KE = \frac{1}{2} m v^2 \).
Potential Energy
\( PE = m g h \).
Conservation
Total energy constant.
Power
\( P = W/t \), watt.
Negative Work
Opposite to displacement.
Joule
1 N·m.
Important Facts
1 J
1 N × 1 m
1 W
1 J/s
KE
Motion
PE
Position
g
10 m/s²
Questions and Answers from Chapter
Short Questions (1 Mark)
Q1. When do we say that work is done?
Answer: Force acts and displacement occurs.
Q2. Write an expression for work when force acts in direction of displacement.
Answer: \( W = F s \).
Q3. Define 1 J of work.
Answer: 1 N force displaces 1 m.
Q4. What is the kinetic energy of an object?
Answer: Energy due to motion.
Q5. Write expression for kinetic energy.
Answer: \( KE = \frac{1}{2} m v^2 \).
Q6. What is power?
Answer: Rate of doing work.
Q7. Define 1 watt of power.
Answer: 1 J/s.
Q8. A lamp consumes 1000 J in 10 s. What is its power?
Answer: 100 W.
Q9. Define average power.
Answer: Total work/total time.
Q10. What is the unit of energy?
Answer: Joule.
Q11. Forms of energy?
Answer: Kinetic, potential, heat, chemical.
Q12. Work done by gravity on satellite?
Answer: Zero (perpendicular).
Q13. Displacement without force?
Answer: Possible (e.g., inertia).
Q14. Holding hay 30 min: Work done?
Answer: No (no displacement).
Q15. Energy in 250 units?
Answer: 2.5 × 10^6 kJ? Wait, 250 kWh = 9 × 10^8 J.
Q16. 40kg at 5m PE?
Answer: 2000 J (g=10).
Q17. Falling object KE at half-way?
Answer: Half initial PE.
Q18. Pendulum comes to rest why?
Answer: Friction dissipates to heat.
Q19. Work to stop mv object?
Answer: \( \frac{1}{2} m v^2 \).
Q20. 1500kg car 60km/h stop work?
Answer: 138889 J.
Medium Questions (3 Marks)
Q1. Look at activities: Is work done? Suma swimming.
Answer: Yes, force by water/Suma displaces her.
Q2. Donkey carrying load: Work done?
Answer: No on load (no vertical displacement); yes by donkey on ground.
Q3. Wind-mill lifting water: Work?
Answer: Yes, wind force displaces water up.
Q4. Green plant photosynthesis: Work?
Answer: No direct force/displacement; chemical work.
Q5. Engine pulling train: Work?
Answer: Yes, engine force displaces train.
Q6. Food grains drying: Work?
Answer: No, evaporation without applied force.
Q7. Sailboat moving by wind: Work?
Answer: Yes, wind force displaces boat.
Q8. Object thrown curved path: Work by gravity?
Answer: Zero net (initial/final horizontal same height).
Q9. Battery lights bulb: Energy changes?
Answer: Chemical to electrical to light/heat.
Q10. 20kg mass velocity 5 to 2 m/s: Work by force?
Answer: Change in KE = ½×20×(4-25) = -210 J (negative).
Q11. 10kg horizontal A to B: Work by gravity?
Answer: Zero (displacement perpendicular to gravity).
Q12. Falling object PE decreases: Violate conservation?
Answer: No, converts to KE; total constant.
Q13. Bicycle riding: Energy transformations?
Answer: Chemical (food) to mechanical (pedal) to KE/heat.
Q14. Push huge rock, no move: Energy transfer?
Answer: No transfer; becomes heat/internal energy in body.
Q15. 250 units energy in joules?
Answer: 250 kWh = 250 × 3600000 = 9 × 10^8 J.
Q16. 40kg 5m PE; half-way KE?
Answer: PE=2000J; half-way KE=1000J.
Q17. Work by gravity on satellite?
Answer: Zero (circular path, perpendicular).
Q18. Displacement without force possible?
Answer: Yes, due to inertia (Newton's first law).
Q19. Holding hay tired: Work done?
Answer: No work on hay; energy to muscles.
Q20. 1500W heater 10h energy?
Answer: 1500 × 36000 = 5.4 × 10^7 J.
Long Questions (6 Marks)
Q1. Illustrate conservation by pendulum oscillation.
Answer: Pulled bob has PE, released converts to KE at bottom, back to PE at other side. Total mechanical energy constant if no friction. Comes to rest as energy dissipates to heat/sound via air resistance. Not violation; transforms to non-mechanical forms.
Q2. mv object constant v: Work to stop?
Answer: To stop, apply retarding force over distance s where \( F s = \frac{1}{2} m v^2 \). Work negative, equals loss in KE. Derivation from equations of motion.
Q3. Calculate work to stop 1500kg car 60km/h.
Answer: v=60/3.6=16.67 m/s; KE=½×1500×(16.67)^2=208333 J. Work= -208333 J (negative to stop).
Q4. Force F on m, west-east displacement: Work positive/negative/zero? (Diagrams)
Answer: Case1: Parallel positive; Case2: Perpendicular zero; Case3: Opposite negative. Explain with \( \cos \theta \).
Q5. Soni: Acceleration zero with forces? Agree?
Answer: Yes, net force zero (balanced forces). Examples: Book on table (gravity=normal).
Q6. 10h four 500W devices energy?
Answer: Total power 2000W; 2000×36000=7.2×10^7 J.
Q7. Falling object stops: What to KE?
Answer: Converts to heat/sound/deformation on impact; conservation holds as transforms forms.
Q8. Pair bullocks 140N plough 15m: Work?
Answer: 140×15=2100 J. Explain force on plough displaces soil.
Q9. KE of m=5kg v=5m/s=25J: Double v KE? Triple?
Answer: Double: 100J (×4); Triple: 225J (×9). Since proportional v².
Q10. 12kg PE=480J g=10: Height?
Answer: h=480/(12×10)=4m. From PE=mgh.
Q11. Two girls 400N climb 8m: A 20s, B 50s power?
Answer: Work=3200J; A:160W, B:64W. Faster = higher power.
Q12. 50kg boy 45 steps 15cm in 9s g=10: Power?
Answer: h=6.75m, W=337.5J, P=37.5W. Calculation step-by-step.
Q13. Object thrown angle: Gravity work?
Answer: Net zero as heights same; detailed path analysis.
Q14. Battery-bulb: Describe changes.
Answer: Chemical in battery to electrical current to heat/light in filament.
Q15. 20kg 5 to 2 m/s: Work?
Answer: ΔKE=½×20(4-25)=-210J. Negative work by force.
Q16. 10kg horizontal: Gravity work?
Answer: Zero, as displacement ⊥ gravity; cos90=0.
Q17. Falling PE decrease: Violate?
Answer: No, PE to KE; total constant per conservation law.
Q18. Bicycle: Transformations?
Answer: Chemical (muscles) to mechanical (pedals) to KE (motion) + heat (friction).
Q19. Push rock fail: Energy where?
Answer: No work on rock; your energy to heat in muscles/body.
Q20. Pendulum: Energy changes, why rest?
Answer: PE to KE and back; friction converts to heat, not violation.
Interactive Knowledge Quiz
Test your understanding of Work and Energy
Quick Revision Notes
Work & Units
- \( W = F s \), 1J=1Nm
- Positive/Negative/Zero
Energy
- KE: \( \frac{1}{2}mv^2 \)
- PE: \( mgh \)
- Conservation
Power
- \( P = W/t \), 1W=1J/s
Exam Strategy Tips
- Derive formulas
- Calculate numerically
- Explain conservation
- Activities describe
- Units consistent
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