NCERT Class 9 Chapter 11 WORK AND ENERGY Notes
NCERT Class 9 Chapter 11 WORK AND ENERGY Notes
Introduction to Work and Energy
The concepts of effort, energy, and power are thoroughly covered in Class 9 Chapter 11, "Work and Energy." In daily life, we refer to any productive physical or mental activity as work, but scientists have a distinct definition of the term. The amount of work a force does on an item is determined by multiplying its magnitude by the distance it moves in the force's direction. There is no direction and only magnitude to work. Similarly, we use the term "energy" a lot in our daily lives, yet science defines it differently. According to physics, energy is the quantifiable quality that is transmitted to a physical system or body and is evident in the production of heat and light as well as in the execution of work.
Work
Work done on an object is defined as the product of the magnitude of the force acting on the body and the displacement in the direction of the force. W = F.s. The SI unit of force is Newton.
11.1.2 SCIENTIFIC CONCEPTION OF WORK
Understanding Work in Science:
- Work is defined as the product of force and displacement.
- To consider work done, two conditions must be met:
- A force must act on an object.
- The object must be displaced.
Examples of Work:
- Pushing a pebble across a surface involves exerting force on it, causing it to move through a distance. Work is done in this situation.
- A girl pulling a trolley applies force, causing it to be displaced. Work is done here as well.
- Lifting a book requires applying force to raise it through a height. Since the book is displaced, work is done in this scenario.
Conditions for Work:
- Both force and displacement are necessary for work to be done.
- If either condition is absent, no work is done according to scientific understanding.
Bullock Cart Scenario:
- In the case of a bullock pulling a cart, although the cart moves and force is exerted on it, work is not necessarily done.
- This is because the force exerted by the bullock may not be in the direction of the displacement of the cart. Therefore, work is not done in this situation.
11.1.3 WORK DONE BY A CONSTANT FORCE
Definition of Work in Science:
- Work is defined as the product of force and displacement when the force acts in the direction of the displacement.
Mathematical Expression:
Work (W) = Force (F) × Displacement (s)
𝑊=𝐹×𝑠
W=F×s
Unit of Work:
- The unit of work is the newton-meter (N m) or joule (J).
- 1 joule is the amount of work done when a force of 1 newton displaces an object by 1 meter along the line of action of the force.
Considerations for Work:
- When the force on the object is zero, no work is done.
- Similarly, when the displacement of the object is zero, no work is done.
- For work to be done, both force and displacement must be present.
Positive and Negative Work:
- When the force and displacement are in the same direction, the work done is positive.
- Conversely, when the force and displacement are in opposite directions (180º angle), the work done is negative.
Negative work is denoted by the minus sign in the work equation.
Examples:
When a baby pulls a toy car parallel to the ground, with force and displacement in the same direction, the work done is positive.
If a retarding force is applied opposite to the direction of motion of an object, the work done is negative as it opposes the displacement.
11.2 Energy
Sources of Energy:
- Energy is crucial for life and its demand is increasing. Sources of energy include the Sun, nuclear reactions, Earth's interior, tides, and others.
- Precise Definition of Energy in Science:
- In science, energy has a precise meaning. It refers to the capability of an object to do work.
Examples of Energy:
- When a cricket ball hits a stationary wicket, the wicket moves, demonstrating the transfer of energy.
- Raising an object to a certain height gives it the potential to do work.
- Other examples include a falling hammer driving a nail into wood and winding a toy car, which then moves on its own.
Transfer and Transformation of Energy:
- Objects with energy can exert force on others, transferring energy to them.
- The receiving object may move and perform work, indicating that any object with energy can do work.
Measurement of Energy:
- Energy is measured in joules (J), the same unit as work.
- 1 joule of energy is required to do 1 joule of work.
- Sometimes, a larger unit called kilojoule (kJ) is used, where 1 kJ equals 1000 J.
11.2.1FORMS OF ENERGY
- Energy and Its Forms
- Mechanical Energy: Sum of potential and kinetic energy.
- Heat Energy: Energy in the form of heat.
- Chemical Energy: Energy stored in chemical bonds.
- Electrical Energy: Energy carried by electrical currents.
- Light Energy: Energy in the form of light.
- Definition: Energy possessed by an object due to its motion.
- Examples: Moving bullet, blowing wind, rotating wheel, speeding stone.
- Factors:
- Increases with speed.
- An object moving faster can do more work than a slower one.
Potential Energy (PE)
- Definition: Energy stored in an object due to its position or configuration.
- Types:
- Elastic Potential Energy: Stored when an object is stretched or compressed (e.g., rubber band, bow).
- Gravitational Potential Energy: Energy due to an object's position above the ground.
- 𝑃𝐸=𝑚𝑔ℎ
- PE=mgh
- m: Mass, g: Gravitational acceleration, h: Height.
- Stretched rubber band.
- Compressed spring.
- Raised object.
Energy Conversion
- Interconvertibility: Energy can be transformed from one form to another (e.g., potential to kinetic).
- Examples:
- A bow releases an arrow: Elastic potential energy transforms into kinetic energy.
- A falling object: Gravitational potential energy converts to kinetic energy.
Law of Conservation of Energy
- Principle: Energy cannot be created or destroyed, only transformed.
- Application:
- Free Fall Example:
- At height ℎ initial PE = 𝑚𝑔ℎ
- KE = 0.
- As it falls, PE decreases, KE increases.
- Just before hitting the ground: PE = 0, KE = maximum.
- Total mechanical energy (PE + KE) remains constant:
11.3 Rate of Doing Work
Power
Definition: Power is the rate of doing work or the rate of transfer of energy.
Average Power
- Definition: Average power is the total energy consumed divided by the total time taken.
- Importance: Useful when the power varies over time.
- Example Calculation
- Scenario: Two girls, each weighing 400 N, climb up a rope through a height of 8 m. Girl A takes 20 s, and Girl B takes 50 s.
Activity: Calculating Potential and Kinetic Energy
Object: 20 kg mass dropped from a height of 4 m.
Gravitational Acceleration (g): 10 m/s².
Calculation Table:
Commercial Unit of Energy
- Kilowatt Hour (kWh):
- Definition: The energy used in one hour at the rate of 1000 J/s (1 kW).
- Conversion:
- 1 kWh = 1 kW × 1 h
- 1 kWh = 1000 W × 3600 s
- 1 kWh = 3.6 × 10⁶ J
- Usage:
- Commonly used in households, industries, and commercial establishments.
- Electrical energy consumption is often expressed in kilowatt hours (kWh), also referred to as 'units'.
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