NCERT Solutions for Class 9 Science Chapter 10 Work and Energy
NCERT Solutions for Class 9 Science Chapter 10 Work and Energy covers scientific concepts of work, positive and negative work, kinetic energy, potential energy, mechanical energy, the law of conservation of energy, rate of doing work (power), and the commercial unit of electrical energy.
Overview of Topics & Subtopics (Click to Jump)
1. Work & Conditions for Work Done
Short Note:
• Work: Work is said to be done when a force applied on an object produces displacement in the direction of the force.
- Formula: Work Done (W) = Force (F) × Displacement (s)
- SI Unit: Joule (J) or Newton-metre (N·m). 1 Joule is the work done when a force of 1 N displaces an object by 1 m in the direction of the force.
• Conditions for Work to be Done:
1. A force must act on the object.
2. The object must undergo displacement.
• Types of Work Done:
- Positive Work: Force and displacement are in the same direction.
- Negative Work: Force and displacement are in opposite directions (e.g., friction).
- Zero Work: Force and displacement are perpendicular to each other, or displacement is zero.
2. Forms of Energy: Kinetic & Potential Energy
Short Note:
• Energy: The capacity of a body to do work. SI Unit: Joule (J). Scalar quantity.
• Kinetic Energy (Eₖ): Energy possessed by an object due to its motion.
- Formula: Eₖ = ½ × m × v² (where m = mass, v = velocity).
• Potential Energy (Eₚ): Energy possessed by an object due to its position, shape, or configuration.
- Gravitational Potential Energy: Energy stored in an object raised to a height (h) above the ground.
- Formula: Eₚ = m × g × h (where m = mass, g = acceleration due to gravity, h = height).
• Mechanical Energy: Sum of Kinetic Energy and Potential Energy of a system.
3. Law of Conservation of Energy
Short Note:
• Law of Conservation of Energy: Energy can neither be created nor destroyed; it can only be transformed from one form to another. The total energy of an isolated system always remains constant.
- Formula: Potential Energy + Kinetic Energy = Constant (Mechanical Energy)
- Free Fall Example: As an object falls, its potential energy gradually decreases while its kinetic energy increases by an equal amount, keeping total mechanical energy constant at every point.
4. Rate of Doing Work (Power) & Commercial Unit
Short Note:
• Power (P): The rate of doing work or rate of energy transfer.
- Formula: Power (P) = Work Done (W) / Time (t) or P = Energy / Time
- SI Unit: Watt (W) or Joules per second (J/s). 1 Watt = 1 J/s.
- 1 Kilowatt (kW) = 1000 W
• Commercial Unit of Electrical Energy: Kilowatt-hour (kWh), commonly called 1 'unit'.
- 1 kWh = 1 kW × 1 hour = 1000 W × 3600 s = 3.6 × 10⁶ Joules (J)
Intext Questions - Page Number: 115
Given:
• Force (F) = 7 N
• Displacement (s) = 8 m
• Work Done (W) = Force × Displacement = F × s
• W = 7 N × 8 m = 56 J
Intext Questions - Page Number: 119
Expression:
Eₖ = ½ × m × v²Where m is the mass of the object and v is its uniform velocity.
Intext Questions - Page Number: 125
• Power: Power is defined as the rate of doing work or the rate at which energy is consumed or transferred.
• 1 Watt: Power is said to be 1 Watt when work is done at the rate of 1 Joule per second (i.e.,
1 W = 1 J/s).
Textbook Chapter End Exercise Questions
(a) Suma is swimming in a pond: Work is done. She applies force against water to move forward.
(b) A donkey is carrying a load on its back: No work is done in moving forward against gravity because the lifting force is vertical while displacement is horizontal (force and displacement are perpendicular to each other).
(c) A wind-mill is lifting water from a well: Work is done. Force acts against gravity in the direction of water movement.
(d) A green plant is carrying out photosynthesis: No mechanical work is done since there is no displacement of an object.
(e) An engine is pulling a train: Work is done. Force applied by the engine causes displacement of the train in the direction of force.
(f) Food grains are getting dried in the sun: No work is done because there is no displacement.
(g) A sailboat is moving due to wind energy: Work is done. Wind force causes the sailboat to move in the direction of wind.
Given:
• Power (P) = 1500 W = 1.5 kW
• Time (t) = 10 h
• Energy consumed = Power × Time
• Energy = 1.5 kW × 10 h = 15 kWh (or 15 units)
In Joules: 15 × 3.6 × 10⁶ J = 5.4 × 10⁷ Joules
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.