If there are non-conservative forces within the system, mechanical energy is transformed into internal energy.


  • $\begingroup$ What system are we talking about? $\endgroup$
    – Steeven
    Jan 5, 2017 at 19:46
  • $\begingroup$ It depends very much on the nature of the non-conservative force. It can involve formation and breaking of weak chemical bonds, turbulence, compressional heating, etc. In the end, does it really matter? $\endgroup$ Jan 6, 2017 at 1:54

1 Answer 1


If there are non conservative forces say for eg : a moving block against friction , then work will have to be done to overcome the friction. Or the friction will do negative work on the body. If the body is moving in a straight line so that change in P.E =0 , we will find that thenrate at which energy being supplied to the block externally (which causes it to move or perhaps accelerate is you consider the example of block sliding down an incline under frictional effects ) is not equal to to the rate at which friction removes energy from the body ( since friction opposes motion ). But the total energy is conserved ( experimental experience and theoretical deduction from Noether's Theorem) , so the energy goes somewhere. The body gets heated up and looses energy by conduction and radiation. It's all because the total energy has to be conserved. Perhaps a better question would have been why the energy gets converted to heat only ( off course besides sound ). But then that's the way nature works


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