# 9th Class Physics Chapter 6 Notes

Work is said to be done when a force acting on a body moves it in the direction of the force.

work = W

SI unit of work is joule (J)

When we say that a body has energy, we mean that it has the ability to do work. SI unit of energy is also joule, the same as work.

Energy exists in various forms such as mechanical energy, heat energy, light energy, sound energy, electrical energy, chemical energy and nuclear energy etc. Energy from one form can be transformed into another.

The energy possessed by a body due to its motion is called kinetic energy.

The energy possessed by a body due to its position is called potential energy.

Energy cannot be created nor destroyed, but it can be converted from one form to another.

Processes in nature are the result of energy changes. Heat from the Sun causes water of oceans to evaporate to form clouds. As they cool down, they fall down as rain.

Einstein predicted the interconversion of matter and energy by the equation E = mc2.

Fossil fuels are known as non renewable resources because it took millions of years for them to attain the present form.

Sunlight and water power are the renewable resources of energy. They will not run not like coal, oil and gas.

Environmental problems such as polluting emission consisting of noise, air pollution and water pollution may arise by using different sources of energy such as fossil fuels, nuclear energy

The ratio of the useful work done by a device or machine to the total energy taken up by it is called its efficiency.

Power is defined as the rate of doing work.

The power of a body is one watt which is doing work at the rate of one joule per second.