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.
When you try to push a wall you do not do any work as distance moved by the body is zero.
If force and displacement are perpendicular to each other then work done by force is zero.
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 unit of energy is also Joule. Energy can neither be created nor destroyed but it can be converted from one form to another form.
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.
Kinetic energy: The energy possessed by a body due to its motion.
KE = ½mv2
Here, m is mass and v is velocity of the body.
Potential energy: The energy possessed by a body due to its position.
PE = mgh
Mechanical energy: The energy possessed by a body due to position or motion is called mechanical energy.
Mechanical energy = PE + KE
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.
Kilowatt hour is a unit of energy. Power is measured in watt or kilo watt.
1 k watt = 1000 watt, 1 kwh = 3.6 x 106 Joule