Preface


This section discusses differential equation used to model pumping swings.

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Pumping swing


You remember that a way to increase the amplitude of the swing oscillations is to perform a characteristic “up-and-down” movement of the center-of-mass of your body as the swing oscillates. You do that twice every period: going forward, and also backward. This corresponds to a driving period T = T0/2, where T0 is the natural period T0 = 2π/ω0, hence a frequency Ω1 = 2ω0. But when you were a little boy, your father would push you, usually once every period, hence with Ω2 = ω0. More generally, you could drive the system by “pushing” only once everyn half-periods, hence with a driving frequency
\[ \Omega_n = \frac{2\omega_0}{n} = \frac{2}{n} \sqrt{\frac{g}{\ell}} , \qquad \mbox{with} \quad n= 1,2,3,\ldots . \]
These are the “resonant” driving frequencies of the ordinary pendulum, where ω0 is its natural frequency of pendulum with length ℓ.

 

 

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