Hi! Work is defined as [tex]dW = F\cdot dr[/tex] so there is no work required to keep things spatially fixed in a gravitational potential. However, consider a hovering helicopter. Even though it is not moving in the gravitational field, it will eventually run out of fuel. Ofcourse there are dissipative losses but obviously it takes energy to keep the chopper from falling down. Or when I'm flying my jetpack, I can't stay put at 100 ft indefinitely, it costs energy to keep hovering - still no work is being done. Where does the energy go?