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SAI INTERNATIONAL SCHOOL
PHYSICS
CLASS IX
Chapter:WORK, ENERGY POWER

LESSON NOTES

What is Energy?

Energy is the ability to perform work. Energy can neither be created nor destroyed. It
can only be transformed from one kind to another. The unit of Energy is same as of
Work i.e. Joules. Energy is found in many things and thus there are different types of
energy.

All forms of energy are either kinetic or potential. The energy in motion is known as
Kinetic Energy whereas Potential Energy is the energy stored in an object and is
measured by the amount of work done.

Types of energy
Some other types of energy are given below:

 Mechanical energy
 Mechanical wave energy
 Chemical energy
 Electric energy

 Magnetic energy
 Radiant energy
 Nuclear energy
 Ionization energy
 Elastic energy

 Gravitational energy
 Thermal energy
 Heat Energy

Unit of energy
The SI unit of energy is joules (J), which is named in honour of James Prescott
Joule.

Mechanical Energy
The energy used to displace a body or to change the position of the body or to
deform the body is known as mechanical energy.

Mechanical energy is of two types (i) Potential energy (ii) Kinetic energy.
Check the detailed work-energy theorem derivation given below.