a pendulum with its bob hanging directly below the support pivot is at a stable equilibrium point; there is no torque on the pendulum so it will remain motionless, and if displaced from this position will experience a restoring torque which returns it toward the equilibrium position. a pendulum with its bob in an inverted position, supported on a rigid rod directly above the pivot, 180° from its stable equilibrium position, is at an unstable equilibrium point. at this point again there is no torque on the pendulum, but the slightest displacement away from this position will cause a gravitation torque on the pendulum which will accelerate it away from equilibrium, and it will fall over.