The definition of kinetic energy change is the resultant force acting on an object, so we pay attention to the difference of kinetic energy before and after doing work, that is, the final kinetic energy-initial kinetic energy. Potential energy is a definite state quantity. Potential energy is that an object overcomes a system force (such as gravity, gravity and electric field force) to do work and has a displacement opposite to the force direction. Therefore, the change of potential energy = work done by the system force =F*L, but at the same time, the calculation of this work is equal to the reduction of two potential energy points relative to the zero potential energy surface (L is only the displacement between two points), and the reduction of these two potential energy points is the initial potential energy-the final potential energy.