Some of the energy is converted to other forms because of friction, therefore, slowing the machine's output work down regardless of any situation. This is the reason that the output work is never 100% of the input.
Work is energy. Energy can't be created or destroyed. Wherever you see energy appearing, it must have come from somewhere. If you see 10 units of energy go INto a machine and 11 units of energy come out, then 1 unit must have been created inside the machine. But that's not possible.
