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Solving Problems in Dynamics and Vibrations Using MATLAB ...

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21<br />

2. Simple Pendulum<br />

θ<br />

l<br />

m<br />

Example<br />

Compute <strong>and</strong> plot the l<strong>in</strong>ear response of a simple pendulum hav<strong>in</strong>g a mass of 10 grams <strong>and</strong> a<br />

length of 5 cms. The <strong>in</strong>itial conditions are θ (0) = 90° <strong>and</strong> θ (0) = 0<br />

Also compare the generated plot with the nonl<strong>in</strong>ear plot.<br />

Solution<br />

The differential equation of motion for the simple pendulum without any damp<strong>in</strong>g is given by<br />

.<br />

..<br />

g<br />

θ = ( − ) s<strong>in</strong>θ<br />

l<br />

If we consider the system to be l<strong>in</strong>ear, i.e., for small angles,<br />

s<strong>in</strong>θ =θ<br />

So the l<strong>in</strong>earized version of the above non-l<strong>in</strong>ear differential equation reduces to<br />

..<br />

g<br />

θ = ( − )θ<br />

l<br />

The above equation is a second order, constant-coefficient differential equation. In order to use<br />

<strong>MATLAB</strong> to solve it, we have to reduce it to two first order differential equations as <strong>MATLAB</strong><br />

uses a Runge-kutta method to solve differential equations, which is applicable only for first order<br />

differential equations.<br />

Let<br />

θ =<br />

y(1)<br />

.<br />

θ<br />

=<br />

y(2)

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