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The Energy is a quantity that is often understood as the ability a physical system has to do work on other physical systems.Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

The total energy contained in an object is identified with its mass, and energy (like mass), cannot be created or destroyed. When matter (ordinary material particles) is changed into energy (such as energy of motion, or into radiation), the mass of the system does not change through the transformation process. However, there may be mechanistic limits as to how much of the matter in an object may be changed into other types of energy and thus into work, on other systems. Energy, like mass, is a scalar physical quantity. In the International System of Units (SI), energy is measured in joules, but in many fields other units, such as kilowatt-hours and kilocalories, are customary. All of these units translate to units of work, which is always defined in terms of forces and the distances that the forces act through.

Potential Energy


Lightning is the electric breakdown of air by strong electric fields, which produce a force on charges. When these charges move through a distance, a flow of energy occurs. The electric potential energy in the atmosphere then is transformed into thermal energy, light, and sound, which are other forms of energy.

Energy Introduction

  Energy is a very common term. Without energy we cannot do any work. Energy is the considered as the tool for doing work. The energy is present in various forms in various place. There is a law formulated for the conservation of the energy. The energy is measured by using the unit joule. The unit of the energy and the unit of work are same.

Forms of Energy

Various forms of energy:

  •  Mechanical energy
  • Heat energy
  • Chemical energy
  • Light energy
  • Sound energy
  • Electrical energy
  • Solar energy
  • Magnetic energy

Mechanical energy:

                Mechanical energy is the energy possessed by vitue of  motion of the body. There are two parts in the mechanical energy kinetic energy and potential energy. Kinetic energy is the form of energy possessed by the motion of the body. Potential energy is possessed by the vitue of its position. Mechanical energy is the energy obtained by the sum of both the potential and kinetic energy.

Heat energy:

                Heat energy is the energy possessed by the hot body when it feels hotness. It is one of the various forms of energy.  

Chemical energy:

                It is one of the various forms of energy.  Chemical energy is the energy possessed from the fuels and domestic gas. This is used as conservation for running vehicles.

Light energy:

                Energy which posses the sensation for seeing the objects is referred as light energy. This is also one of the various forms of energy.

Sound energy:

            Energy which posses the sensation for hearing the sound is referred as sound energy.

Electrical energy:

                Energy which is carried out by electric current is referred as electrical energy.

Solar energy;

                Energy which is radiated from the sun is defined as solar energy.

Magnetic energy:

                The magnetic field is produced around the magnet. Energy which is stored by the magnetic field is referred as magnetic energy.

Law of Conservation of Energy

Law of conservation of energy:                             

                The total amount of energy in the isolated system is maintained as constant when energy is transformed from one form to another. The law of conservation of energy is defined as,

                Energy cannot be created or destroyed but it can be transformed the energy from one form to another form.

                The law of conservation of energy is based on the experience. The energy which is defined here must be considered for experiencing the law of conservation of energy. In alternative the law of conservation is defined as, In universe the total energy remains constant.  

Work Power and Energy

A person goes to his office by walk, sits at his office desk from 10 a.m to 5 p.m ., does his work and then finally returns to his home by walk. But according to physics, he does not do any work sitting in his office. He does work only during his walk to his office and back to his home.

Work is said to be done by a force when a body undergoes displacement parallel to the line of action of the force .


It is measured by the scalar product of the vectors representing the force applied on a body and the displacement of the body due to the force. The work W due to force F on a body that produces a displacement S is given by

                W  =  F . S 

                W = FS cosimg     ,  where S cosimg  is the component of displacement parallel to the line of action of the force F ,                                              img is the angle between the force and displacement vectors.

                                   Work is a scalar quantity whose SI unit is joule (J) .  Its dimensional formula is [ML-1T-2] .

      Work may be positive or negative. Work is positive if   0 `<=`  img  `<=`  900 .  Work is negative if 900 <= img  <= 1800 .

                    Work is zero if (i) the displacement is zero. A person holding a brief case and waiting for a bus makes no displacement.  So, no work is done by him. Similarly a person sitting at a place does not work . (ii) the angle between the force and displacement is 900 . When the moon revolves round the earth in a circular orbit, the angle between the gravitational force of attraction of the earth on the moon that causes the centripetal force for the circular motion and the  displacement is 900 . When we walk on a horizontal road work is done by us. But the work done by gravitational force is zero since the displacement is perpendicular to the gravitational force . (iii) the force is zero. When an object moves on a smooth horizontal surface force in the direction of displacement is zero and the work done during this displacement is zero.

        If the displacement is in the same direction of force , img = 00  and work done is       W  =  F S

            This formula for work can be used when the force is uniform throughout its displacement i.e., the force is a constant force. If the force is variable,  

                           W  =  `img         , where dx is the displacement under the action of force F.

     The same can also be obtained by the area under force - displacement graph :


Work Power and Energy : Power

More often the rate at which the work is done, is more important than doing work. To extinguish the fire in a building water is to be lifted up and sprayed on the fire as fast as possible. When water is filled int eh overhead tank of a building, how fast is it filled depends upon the motor used to lift the water into the tank.

 The time rate of doing work is called power.

    If W is the work done by a force in a time interval t then the average power     Pimg = W / t . 

    The instantaneous power is   P  =  dW / dt .

    The work done by a force F for a displacement dr is ,        dW  = F . dr

                                  P = dW / dt  =  F . dr / dt    =  F . v    =  F v cosimg  , where v is the instantaneous velocity when the force is F .

          Power is a scalar quantity. Its SI unit is watt (W) and dimensional formula is [ML2 T-3 ] . Another unit of power in use to describe the power of motors and automobiles is horse power(hp) .  1 hp = 746 W .

           The electrical unit used to measure the consumption of electricity in our houses is kilo watt hour (kwh) is not the unit of power. It is the unit of electrical energy consumed.

                                      1 kilo watt hour = 1000 watt hour  =  1000 J / S x 3600 S  =  3.6 x 106 J .

Work Power and Energy : Energy

 When an animate or inanimate body is able to do work we say that it possesses energy.

                                    Energy of a body is its capacity to do work.

A moving vehicle possesses energy since it does work against friction and air resistance before coming to rest after the engine is switched off. Water stored in a dam has energy since it can run the turbines when the water flows down onto the turbines.

       The different forms of energy are mechanical energy, light energy, heat energy, a sound energy, electrical energy, nuclear energy etc.    In mechanics, we are interested in mechanical energy. A body possess mechanical energy in two different forms. (1) By virtue of its motion which is called kinetic energy. (2) By virtue of its position or configuration which is called potential energy.

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