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# Lakhmir Singh Solutions for Class 9th Physics Chapter 4: Work and Energy

The chapter work and energy revolve around work done by an object when force is applied on it, the energy produced, and the rate of doing work, i.e. power. Work is said to be done when the force applied produces motion. Therefore, it is a scalar quantity, and the SI unit is Joule.

Work done by any kind of force can give various results:

a). Positive; when a force is acted in the direction of motion of the body.
b). Negative; when a force acts opposite to the direction of motion.
c). Zero; when a force acts at a right angle.

Here, students will learn how to calculate work done under the following conditions:

1. When force is acting obliquely.
2. When the body moves at an angle to the direction of a force.
3. When the force acts at right angles to the direction of motion.
4. When the force that acts upon a body is opposite to that of the body's motion.

The chapter further explains the basics of energy and different forms of energy, including kinetic energy, produced due to motion of a body, potential energy produced due to position of a body, chemical energy, heat energy, sound energy, light energy, electric and nuclear energy.

Power is defined as the rate at which work is done or the rate at which energy is consumed. Its SI unit is Watt. 1 Watt is defined as the power of an appliance at which the rate is one Joule per second. Kilowatt, Megawatt, and horsepower are other units of power

This chapter includes some practical concepts which can be used in daily life, like the relation of a kilowatt-hour, the commercial unit of energy, and Joule. For example, when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour, one kilowatt-hour of electrical energy is consumed.

Energy cannot be created or destroyed. It can only be transformed from one form to another by the process of transformation of energy. This energy transformation can be used to generate electricity at a hydroelectric power plant and thermal power plants. Electric iron, steam engines are certain devices used as energy converters. The total energy remains constant during transformation. This can be illustrated by taking the example of energy conservation in a simple pendulum or energy conservation during the free fall of a body.