Important Terms

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

• Work = FS
• SI unit of work is the joule (J).

1. When we say that a body has energy, we mean that it can 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, 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.

Einstein predicted the interconversion of matter and energy by the equation $E = mc^{2}$.

Fossil fuels are known as nonrenewable resources because it took millions of years for them to attain their present form.

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. 

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