Class 9 Science Notes | Work and Energy Notes

Class 9 Science: Work and Energy

Introduction

In the study of physics, the concepts of work and energy are fundamental in understanding how forces interact with objects and how these interactions cause changes in motion and state. These concepts have wide applications, ranging from simple daily activities to complex industrial processes.

Work

Definition: In physics, work is done when a force causes displacement of an object. The amount of work done is calculated using the formula: $\text{Work (W)} = \text{Force (F)} \times \text{Displacement (d)} \times \cos(\theta)$ where:

• $W$ is the work done,
• $F$ is the force applied,
• $d$ is the displacement of the object,
• $\theta$ is the angle between the force and the displacement vector.

Units: The SI unit of work is the joule (J). One joule is the work done when a force of one newton displaces an object by one meter in the direction of the force.

Conditions for Work:

• A force must be applied.
• The object must be displaced.
• There must be a component of the force in the direction of displacement.

Types of Work:

1. Positive Work: When the force and the displacement are in the same direction, work is positive. For example, pushing a cart forward.
2. Negative Work: When the force and the displacement are in opposite directions, work is negative. For example, frictional force acting against the motion of a sliding object.
3. Zero Work: When the force is perpendicular to the displacement, no work is done. For example, carrying a bag while walking horizontally.

Examples:

• A person lifting a weight from the ground to a shelf performs work as the force (lifting) and displacement (upwards) are in the same direction.
• Pushing a stalled car to move it involves work as the force exerted on the car results in its displacement.

Energy

Definition: Energy is defined as the capacity to do work. It exists in various forms and can be transformed from one form to another but cannot be created or destroyed (Law of Conservation of Energy).

Types of Energy:

1. Kinetic Energy (KE):

   • The energy possessed by an object due to its motion.
   • Formula: $\text{KE} = \frac{1}{2} m v^2$
   • Where $m$ is the mass of the object and $v$ is its velocity.
   • Examples: A moving car, a flying airplane, flowing water.

2. Potential Energy (PE):

   • The energy possessed by an object due to its position or configuration.
   • Gravitational Potential Energy: $\text{PE} = m g h$
     • Where $m$ is the mass, $g$ is the acceleration due to gravity, and $h$ is the height.
   • Elastic Potential Energy:
     • Energy stored in stretched or compressed objects like springs.
   • Examples: A book on a shelf, water stored in a dam, a compressed spring.

3. Mechanical Energy:

   • The sum of kinetic and potential energy in an object.
   • Formula: $\text{Mechanical Energy} = \text{KE} + \text{PE}$

4. Chemical Energy:

   • Energy stored in the bonds of chemical compounds.
   • Examples: Energy in food, fuel, batteries.

5. Thermal Energy:

   • The internal energy in substances—the vibration and movement of atoms and molecules.
   • Examples: Heat energy from the sun, geothermal energy.

6. Nuclear Energy:

   • Energy stored in the nucleus of atoms, released during nuclear reactions.
   • Examples: Energy from nuclear fission in reactors, fusion in the sun.

7. Electrical Energy:

   • Energy from the movement of electrons.
   • Examples: Electricity in homes, lightning.

8. Radiant Energy:

   • Energy carried by electromagnetic waves.
   • Examples: Light energy, X-rays.

Transformation of Energy: Energy can be converted from one form to another. For example:

• A battery (chemical energy) converts to electrical energy to power a device.
• A bulb converts electrical energy into light and heat energy.
• Photosynthesis converts radiant energy from the sun into chemical energy in plants.

Law of Conservation of Energy: This law states that energy cannot be created or destroyed; it can only be transformed from one form to another. The total energy in a closed system remains constant.

Work-Energy Theorem: The work done on an object is equal to the change in its kinetic energy. $W = \Delta KE$ $W = \frac{1}{2} m v_f^2 - \frac{1}{2} m v_i^2$ where $v_f$ is the final velocity and $v_i$ is the initial velocity.

Power

Definition: Power is the rate at which work is done or energy is transferred. $\text{Power (P)} = \frac{\text{Work (W)}}{\text{Time (t)}}$

Units: The SI unit of power is the watt (W). One watt is equal to one joule of work done per second.

Examples:

• A 100-watt light bulb uses 100 joules of energy per second.
• An athlete running up stairs quickly is exerting more power compared to walking up slowly, even if the work done (moving the body to the top) is the same.

Practical Applications

1. Mechanical Systems:

   • Engines convert chemical energy in fuel into mechanical energy.
   • Machines like levers, pulleys, and gears use mechanical energy to perform tasks efficiently.

2. Electrical Systems:

   • Generators convert mechanical energy into electrical energy.
   • Electric motors convert electrical energy into mechanical energy.

3. Thermal Systems:

   • Heat engines, like car engines and power plants, convert thermal energy into mechanical energy.
   • Refrigerators and air conditioners transfer thermal energy to maintain temperature.

4. Renewable Energy Sources:

   • Solar panels convert radiant energy into electrical energy.
   • Wind turbines convert kinetic energy of wind into electrical energy.
   • Hydroelectric power plants convert the potential energy of stored water into electrical energy.

Summary

Understanding work and energy is crucial in the study of physics as it forms the basis for analyzing the interactions and transformations that occur in various physical systems. These concepts not only explain natural phenomena but also underpin the operation of numerous technologies that drive modern society. By comprehending how work and energy interplay, students gain insights into the fundamental principles that govern motion, forces, and the various forms of energy that we utilize in everyday life.