Keiran OConnor said:
friction lose energy when the conservation of energy law doesn't allow energy to be lost ??
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.