Class 9 Science Note

Work, Energy and Power

Work is defined as the product of the force and the distance covered by the body in the direction of force. Mathematically,

W= F*D


The S.I unit of the work is joule.

1 joule work is defined as the amount of work done to cover a distance of 1m in the direction of force when the force of 1N is applied on the body.

 It is a scalar quantity.

 Work is done against two forces

 Work against the gravity.

Work against friction.



 The ability of a body to do the work is called energy.


 There are different forms of energy .They are:


Chemical energy:

Chemical energy is stored in the form of potential energy. This energy can be released by burning the substance.


Mechanical energy:

 It is of two types:

Kinetic energy: The energy possessed by the moving body is called kinetic energy. It is given by 

K.E = ½ mv2 where, m is the mass of the body

 V is the velocity.


Potential energy:

 The energy possessed by the body due to its position is called potential energy. It is given by  P.E = mgh where  m is he mass of the body ,g  is acceleration due to gravity , h is the height of the body from the ground level.


Electrical energy: The energy produced due to the flow of charge is called electrical energy .it is used widely in electric appliances.


Heat energy: It is the energy due to the motion of molecule. Every molecule possesses kinetic energy and the sum of K.E is heat energy. Sun is the main source of heat energy.

 Sound energy: Sound is the form of energy which is produced due to the vibration of molecule.


Light energy:

Sun is the main source of the light energy. It gives the sensation of vision. It is formed through chemical, radiation, and mechanical means. Light energy can be converted into other forms of energy.


 Magnetic energy:

Magnetic energy is used to produce the electricity. Magnetic energy is produced with the help of strong magnets.


Nuclear energy: It is the form of energy produced when two or more nuclei combined to form a heavy nucleus. This process is called nuclear reaction. A huge amount of energy is released during this process.


Transformation of energy: Energy can neither be created nor be destroyed but can be transformed from one form to another form. E.g. production of electricity


Potential energy  à  kinetic energy  à  electrical energy  à  light energy

 Water store in dam à rotation of turbine à generator à  in bulb



Power: The rate of doing work is called power. Its unit is joules per second or watt. Mathematically, power = W/t where W= work done, t = time taken

It is the measure of how fast the work is done.  If 1 joule work is done in 1 sec then it is called 1 watt.

Power is a scalar quantity.

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