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Law of Conservation of Energy

The energy in a system may take on various forms, such as light, sound, thermal, kinetic, potential, heat. 
  • The law of energy of conservation states that “The energy cannot be created nor be destroyed but the only thing that can happen with energy is that it can change from one form to the other form". 
  • It means that the total amount of energy in the system is a constant. Let us try to understand this principle from following examples.
Example 1

  • A light bulb converts electrical energy into light energy. 
  • A 100 W electric bulb produces 80 W light energy. According to the law of conservation of energy the energy cannot be created nor destroyed, then what is about 100 W – 80 W = 20 W ?
law of conservation of energy
  • It is destroyed or the law of energy conservation is wrong! 
  • Not, at all! Think differently. Here 100 W is electric input, 80 W is light output and remaining 100 W - 80 W = 20 W is heat losses. Therefore the law of energy conservation is almost correct.
Example 2

  • The pendulum is shown in the figure B.
  • Mass of the ball m = 5 kg
  • Height h = 0.2 m
  • Gravity g = 9.8 meter / sec2
          Potential energy = mgh
                                  = 5 × 0.2 × 9.8
                                  = 9.8 Joule


  • The potential energy is 9.8 J whereas the kinetic energy becomes zero at position A of the ball. 
  • The kinetic energy becomes maximum ( 9.8 J ) at point B whereas the potential energy becomes zero at position B.  
  • The potential energy again becomes maximum at position C of the ball and kinetic energy becomes zero.
  • We can say that Potential energy (P.E.) + Kinetic energy (K.E.) = 0
         PE = – KE
        Therefore total Mechanical Energy = PE + KE
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