Work: The product of displacement and force in the direction of displacement of a body is called work.
Work = Displacement × Force in the direction of displacement.
The unit of work is Joule in m.k.s. system and erg in C.G..S. system.
1 Joule = 107 erg
Energy: The capacity of a body to do work is called energy.
Energy possessed by a body = Total work that the body can do
The m.k.s. unit of energy is Joule.
Joule: When a body moves through a distance of 1 m under the force of 1 N in the direction of force, then work done by the body is said to be 1 J.
Different Forms of Energy
(i) Kinetic energy: The energy possessed by a body due to its motion is called kinetic energy.
KE = ½ mv2
Here m is mass and v is velocity of the body.
(ii) Potential energy: The energy possessed by a body due to its position is called P.E.
Potential energy = mgh
(iii) Mechanical energy: The energy possessed by a body due to position or motion is called mechanical energy.
Mechanical energy = PE + KE
(iv) Thermal energy: The energy which gives us sensation of warmth is called thermal energy.
(v) Light energy: The energy which helps us see things is called light energy.
(vi) Electrical energy: The energy due to moving charges is called electrical energy.
(vii) Nuclear energy: The energy released in nuclear reactions by conversion of mass into energy is called nuclear energy.
Power
The time rate of doing work is called power.
Power = work done/time
The SI unit of power is Watt. It is also measured in horse power, 1 HP = 746 watt.
Watt: If one Joule of work is done by a body in one second, then power of a body is said to be one watt.