Work, Power And Energy – CBSE Notes for Class 9 Science
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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 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.
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 gum 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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