Work, Net work, Work done by friction

PhanthomJay

Some more food for thought:

The mechanical energy (KE + PE) of an object changes when work is done on it by non-conservative forces. The work done by gravity is already included in the PE term. Even though in the book example there is no net work being done, it is the work done by your applied non-conservative force that changes the books mechanical energy (in this case just a PE change).

The total energy of the earth-book-you system can never change, which includes the energy from the Breakfast of Champions Wheaties you had prior to exerting your force.

Simon Bridge

It makes sense... work is done by something on something else, so you are always considering part of a system.