Does the work-energy theorem hold if friction acts on an object?
Yes. Friction just must be included in the “work”. Remember, when friction acts, negative work is done on the object. Positive work is done as local heating at the surface interface. Friction and human forces are non-conservative forces, meaning that the energy isn’t tracked mathematically. Sure, you could trace the energy, and when studying the conduction of frictional heat, we do trace this energy. But frictional heat and chemical energy of human food aren’t classified as “mechanical energy”. Forces like gravity, springs, and electrostatics are conservative forces, and each have an associated potential energy. A better statement of the energy conservation is the following: PE1_net + KE1 + W_nonconservative = PE2_net + KE2 If no net non-conservative forces act on the body, “mechanical energy” is conserved. “Mechanical energy” being the sum of all potential energy and kinetic energy. A human force acting in the direction of motion of a body produces positive work. A human force acting
