I'm having trouble with the concept of conservative forces. I've been studying things of that nature for awhile now and can make most calculations on my own, but there are a few specific details that confuse me. It's easiest to explain my confusion with two simple examples. First Example: Let's just consider a particle moving in the Earth's gravitational field. Let the potential energy be zero at a height of zero. So, the total potential energy is just mgh. It makes perfect sense that when this particle is moving in the field that energy will be conserved (i.e. an increase in height leads to an increase in potential energy and an equal decrease in kinetic energy). The part that bothers me is thinking about a person lifting an object of mass m a height h. Now, the work that person must do to lift the object is simply mgh. Also, the work done by the gravitational field on that object during the lifting process is -mgh. There is a net work of zero done on the particle (which is good since the kinetic energy didn't change, and there is no violation of the work-energy theorem). The problem is it appears to me that no energy is put into the particle. Whatever energy the person puts into the object, the gravitational field takes out of it (since mgh + -mgh = 0). However, as we all know the potential energy did increase by mgh. Where is this energy coming from? Second Example: Consider a particle of mass m attached to a horizontal spring at it's equilibrium position. In order to determine the potential energy as a function of displacement x, we strech the string and determine the amount of work we had to do on the particle. However, I have the same issue again as above. I did 1/2 kx^2 amount of work on the particle while the sping did -1/2 kx^2 worth of work on the particle (i.e. no net work). But, it's potential energy increased by 1/2 kx^2. Where is this energy coming from? The only answer I can seem to come up with, is the fact that you can only define a potential for conservative forces. Therefore we only set the change in potential energy equal to the work done by the conservative force (i.e. the gravitational force, not the man's force during lifting). The net work done on the object is still zero, so there is no change in kinetic energy. But, the total work done by conservative forces (for the gravitational example) is -mgh (resulting in an increase of mgh in it's potential energy). This whole idea is still slightly confusing. Does energy conservation work out in this case? Can someone explain what is really going on here? It would be great if you could point me to a source to read up on conservative forces that discuss my troubles. Thank you to anyone who replies.