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# Work, Power and Energy Class 9 Notes Science Chapter 11

Facts that Matter

Work: When a force acts on an object and the object shows displacement, the force has done work on the object.
Two conditions need to be satisfied for work to be done:
(i) A force should act on object
(a) The object must be displaced
Work = Force x Displacement Unit of workdone = Joule = Newton x metre 1 Joule work is said to be done when 1 Newton force is applied on an object and it shows the displacement by 1 meter.

Energy
The capacity of a body to do work is called the energy of the body.
Unit of energy = Joules 1KJ = 1000 J

Forms of Energy: The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.

Kinetic Energy: Energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.
Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity

Derivation
Let an object of mass m, move with uniform velocity u, let us displace it by s, due to constant force F, acting on it

Potential Energy
The energy possessed by a body due to its position or shape is called its potential energy.
shape

Gravitational Potential Energy: (GP)
When an object is raised through a height, work is said to be done on it against gravity.
The energy possessed by such an object is called the gravitational potential energy.
GPE = work is done in raising a body from the ground to a point against gravity.
Derivation

Consider a body with mass m, raised through a height h, from the ground, Force required to raise the object = weight of object mg.
The object gains energy to the work done on it.
Work done in both the cases (i) and (ii) is same as a body is raised from position A to B, even if the path taken is different but the height attained is the same.

Mechanical Energy: The sum of kinetic energy and potential energy is called mechanical energy.

Law of Conservation of Energy:
Energy can neither be created nor destroyed, it can only be transformed from one form to another. The total energy before and after transformation remains the same.

Potential energy + Kinetic energy = Constant (Mechanical energy)
A body of mass ‘m’ is raised to height ‘h’ at A its potential energy is maximum and kinetic energy is 0 as it is stationary.
When body falls at B, h is decreasing hence potential energy decreases and V is increasing hence kinetic energy is increasing.
When the body is about to reach the ground level, h = 0, v will be maximum hence kinetic energy –> potential energy
Decrease in potential energy = Increase in kinetic energy
This shows the continual transformation of gravitational potential energy into kinetic energy.

Power

Commercial Unit of Energy

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