Question

Why doesn’t the principle of mechanical energy conservation hold in situations when frictional forces are present?

A.Chemical energy is not simply converted to electrical energy.

B.Kinetic energy is not simply converted to a form of potential energy.

C.Kinetic energy is simply converted to a form of gravitational energy.

D. Potential energy is simply converted to a form of gravitational energy.

Answer 1

It does. What a strange question! Conservation of energy always holds. The only strange thing about friction forces is that the kinetic energy of a macroscopically observable degree of freedom (e.g. the movement of a car) gets transformed into the kinetic and potential energy of bazillions of microscopic unobservable atomic-scale degrees of freedom, e.g. the wiggling of individual atoms or molecules, or the compression or expansion of individual chemical bonds.

I guess the least screwy answer is probably B, because I interpret it to mean that macroscopic kinetic energy is not entirely converted to a form of macroscopic potential energy when frictional forces act. That's a true statement. I'm not sure what "simply" in this sentence means -- that doesn't have any physics meaning of which I am aware.