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B Friction and conservation of energy

  1. May 25, 2016 #1
    How does friction lose energy when the conservation of energy law doesn't allow energy to be lost ??

    Sorry if this is a stupid question I have tried googling but can't find much to help.
  2. jcsd
  3. May 25, 2016 #2
    Friction causes conversion of mechanical energy to heat energy.. The surfaces become warmer. There's no loss of energy.
  4. May 26, 2016 #3


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    If you ever find an apparent breech of conservation of energy it invariably means you have drawn your system boundary in the wrong place (eg you have forgotten a means by which energy can enter or leave your system). In other words your system isn't "closed". Conservation of energy only applies to closed systems.
  5. May 26, 2016 #4
    The confusion is coming from looking at the problem from two different viewpoints. Consider a block of mass M sliding down a frictionless ramp from a height H. The block initially has potential energy ##E_p = MgH##. We know that at the bottom of the ramp all of that potential energy will have been transformed into kinetic energy ##E_k ## resulting in a velocity of ##v=\sqrt{2gH}## at the bottom of the ramp. So we say energy is conserved because the starting energy is equal to the ending energy, or ##E_p - E_k = 0##. So far so good?

    Now consider that the ramp is not frictionless. In this case the velocity must be ##v<\sqrt{2gH}##. The reason we know that the velocity is less than the previous value is that we know energy is always conserved! Some of the energy of the block was transferred to the ramp due to friction in the amount ##E_f##. If we only consider the energy of the block it will seem like some energy disappeared somewhere along the way to the bottom of the ramp. But we know the energy did not disappear, it was both transformed and transferred in the friction case but only transformed in the non friction case. In the friction case we still have ##E_p - E_k - E_f = 0##, and so we see energy is conserved, as always.
    Last edited: May 26, 2016
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