In physics, a conservative force is a force with the property that the net work done in moving a particle between two points is independent of the path taken. Therefore, to determine a force is conservative or not, we can let the force act on a particle that moves along any closed path, start at a position and end at the same position (this way that's literally this is a "round trip".) The force is conservative only if the total energy it transfers to and from the particle during the round trip along any kinds of closed path is zero. The total energy here we are talking about is also called "net work", so we can say: The net work done by a conservative force on a particle moving around any closed path is ALWAYS zero. Otherwise, that's nonconservative force, mechanical energy is lost and is turned into heat (thermal energy). For example, friction force is nonconservative force.