From the standpoint of energy quantity, all is well. In all energy conversions, energy is conserved; no energy is lost. This is best illustrated by a frictionless pendulum that has 100% conversion between potential and kinetic energy. (In a real-world pendulum, not as much kinetic energy is obtained; part of the potential energy is converted to heat, because of friction, but energy is still conserved.)

In position 1 all the energy is potential, in position 2 all potential energy is converted to kinetic energy, and in position 3 all the kinetic energy is converted to potential energy.

From the standpoint of energy quality, however, some energy conversions are 'better' than others. By this we mean that nature imposes a lower 'tax' on certain energy conversions and a higher 'tax' on other energy conversions. The most important example of this fact of life is the conversion between work and heat.

 

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