►With appropriate scalings, Newton’s equation of motion for a pendulum with a mass in a gravitational field constrained to move in a vertical plane at a fixed distance from a fulcrum is
…The periodicity and symmetry of the pendulum imply that the motion in each four intervals and have the same “quarter periods” .
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►If the parameters of a physical system vary periodically with time, then the question of stability arises, for example, a mathematical pendulum whose length varies as .
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