Example #2 (continued from lecture 13) : a block (mass m) slides frictionlessly along a looped track. What is the minimum height h to release it from so that it stays on the track at the top of the loop of radius r. We have to be careful with non-conservative forces: We do not define pe for a non-conservative force. But we can now modify our work-energy principle from w net . Object moving in a circular path? generalized form that includes potential energy . The conservation of mechanical energy is only true if there are acting. We know that we have to apply a force on the spring) Of course, the spring pushes back with an equal and opposite force. (units: n/m) and varies from spring to . Be careful ! the force (f=-kx) is changing as x changes. Power = rate at which work is done. = rate at which energy is expended.