Consider a skater skating on a frictionless parabolic ramp, the force acting on the skater is given by the negative slope of the potential energy curve. At the bottom of the ramp, the force acting on the skater is zero. Consider an imaginary potential energy diagram; the net force acting on the skater will be zero at point x1 and similarly at point x2. Any displacement away from point x1 will result in a restoring force on the skater, directed towards the point x1. Whereas, any displacement of the skater at point x2 will result in a force, directed away from the point x2. Any minimum point in the potential energy curve is a stable equilibrium point, whereas any maximum is an unstable equilibrium point. If the total energy of the skater is E2, then the skater will be able to escape and go beyond point x2. But if the total energy is E1, then the skater will be trapped between positions x0 and x2.