An energy diagram is a plot of potential energy as a function of the position of an object when a conservative force is acting upon it. Because of the net conservative forces on the object, the total mechanical energy of the system is constant. Consider a skater at a rest point A on a frictionless parabolic ramp of height y. The gravitational potential energy of the skater, thus, is equal to mgy, while its kinetic energy is zero. As the skater moves down, its potential energy decreases as x2. At the bottom of the ramp, the potential energy of the skater becomes zero, while its kinetic energy reaches maximum. When the skater moves towards point B, the potential energy reaches a maximum value with the kinetic energy approaching zero. Further, since the ramp is frictionless, the skater performs an oscillatory movement at the bottom of the ramp. Thus, the force acting on the skater is a negative derivative of the potential energy at that given instant.