#### WORK

Work is said to be done,if a force acting on a body is able to actually move it through some distance in the direction of the force is called work.Work is equal to Force multiplied by Distance.
Work =Force ✕ Distance,
i.e., W= F.S
Its SI unit is a joule (J).

#### POSITIVE WORK DONE (EXAMPLES)

When body falls freely under gravitational pull.
When a horse pulls a cart on a level road.

#### NEGATIVE WORK DONE (EXAMPLES)

When a body is made to slide over a rough surface.When a positive charge is moved towards another positive charge.

#### ZERO WORK DONE (EXAMPLES)

When a coolie travels on a platform with a load on his head.
When a body is moved along a circular path with the help of a string.When a person does not move from his position but he may be holding any amount of heavy load.

#### ENERGY

The energy of an object is its capacity for doing work.It is a scalar quantity and its unit is joule (J).
Energy can exist in different forms such as mechanical energy,heat energy,sound energy,light energy etc.
Mechanical energy is of two types i.e., kinetic energy and potential energy.

#### KINETIC ENERGY

The energy possessed by a body by virtue of its motion is called its kinetic energy.
A body of mass m and moving with velocity v has
kinetic energy =1/2 mv= P2/2m
When momentum is doubled kinetic energy becomes four times.

#### EXAMPLES

Kinetic energy of air is used to run wind mills.
Kinetic energy of running water is used to run the water mills.
A bullet fired from a gun can pierce a target due to its kinetic energy.

#### POTENTIAL ENERGY

It is the energy possessed by a body by virtue of its position.
Suppose a body is raised to a height h above the surface of the earth,then potential energy of body = mgh
Also here potential energy = work done = mgh

#### EXAMPLES

The potential energy of the wound spring of a clock is used to drive the hands of the clock.
Due to potential energy of the stretched bow,the arrow goes forward with a large velocity on releasing the bow.
The potential energy of water in dams is used to run turbines in order to produce electric energy using the generators.

#### PRINCIPLE OF CONSERVATION OF ENERGY

Energy can neither be created nor destroyed but can only be transformed from one form to another.
The total amount of energy in the universe remains constant.
The body falls its potential energy decreases but, at the same time kinetic energy increases by an equal amount.

#### TRANSFORMATION OF ENERGY

In a heat engine heat energy changes into mechanical energy.In the Sun, mass changes into radiant energy.
In an electrical heater,the electric energy is converted into heat energy.
In an electric bulb,the electric energy is converted into light energy.In burning coal,oil etc, the chemical energy changes to heat energy.
When we rub our hands, heat is generated.Here mechanical energy of muscles is converted into heat energy.
In an electromagnet, the electric energy is converted into magnetic energy.

#### MASS-ENERGY EQUIVALENCE

In 1905,Einstein proved the relation between mass and energy.
E = mc2, where c is the velocity of light.
E = mc2, leads to verification of the two laws-law of conservation of mass and law of conservation of energy.

#### POWER

The time rate of doing work is called power.
If W is work done in time t then power P, is P = W/t
Power is a scalar quantity with SI unit Watt or Joule/s.
1 watt hour = 3600 joule
1 kilowatt hour = 3.6 ✕ 106 J
1 HP = 746 watt