Partnership for Integration of Computation into Undergraduate Physics

Exercise 1: Using the Euler Cromer method, solve $\ddot{\theta}=-{\omega}^{2}sin{\theta}$ and plot position, $\theta$, vs time, up to a total time of 10 periods, for a simple pendulum with SAA (i.e. $sin{\theta}={\theta}$) and without SAA for initial angles of 5, 15, 30, 45 and 60 degrees (minimal set: 5, 30 and 60 deg). Take $\omega=2\pi$, initial velocity zero, and integration step size of 0.01. Put both plots on same graph and discuss qualitatively how the time behavior of the pendulum’s oscillations change if the SAA is not used. (Students can use PICUP article by K. Roos, Simple Hanging Harmonic Oscillator, as reference for Euler-Cromer method, as well as for behavior of pendulum with SAA) Exercise 2: Extend the code for Exercise 1 to determine the smallest angle (integer multiple of 5 will suffice) for which the average period of the pendulum without SAA exceeds the period of the pendulum with SAA by 5%. You can start with an angle of 5 degrees and increment by 5 degrees. Students can use findpeaks in MATLAB to determine peak amplitudes and then determine period. (Will require students to think about how to get average period since numerical results can have some variations and uncertainties – one way, as shown in sample code, is to use the command mean(diff()) in MATLAB. Another way, depending on student and instructor interest is to use fast Fourier transform (fft) to get mean frequency of oscillations). Exercise 3: Now introduce a damping coefficient and solve $\ddot{\theta}+k{\dot{\theta}}+\omega^{2}sin\theta=0$ by using ODE45 in MATLB. First set $k = 0$ and $\theta = 5$ and make sure that you get correct results for an undamped oscillator by comparing to results of Exercise 1. Then set $k = 0.05$ and compute and plot position vs time for initial angles of 5 and 60, initial velocity zero and for $\omega=2\pi$. Make phase plots of $\dot{\theta}$ vs $\theta$ for damped ($k = 0.05$) oscillator with amplitude of 60 degrees and interpret the plot. ((minimal set: can skip phase plots; Students can consult PICUP article by D. O'Neil, Energy and Phase Space of Damped Oscillator for reference) Exercise 4: Now, using ODE45, model the forced oscillator, given by $\ddot{\theta}+k{\dot{\theta}}+\omega^{2}sin\theta=Fcos(\omega_Ft)$ by choosing suitable values of $k$, $\omega$, $F$ and $\omega_F$. $k$ is the damping rate, $\omega$ is the natural frequency of the oscillator, $\omega_F$ is the frequency of the forcing term and $F$ is the amplitude of the forcing term. (Requires students to think of what values to use). Plot amplitude vs time for a few different parameters. Exercise 5: Consider an oscillator described by $\ddot{\theta}+{\alpha}{\theta}^{3}=0$, where you can take $\alpha = 1$. (Nonlinear oscillator). Using Euler-Cromer method, compute and plot a graph of the period of the oscillator vs 1/amplitude. Take amplitudes from 5 to 85 degrees and initial velocity zero. Does the period vary with amplitude, and how does the variation compare with the simple harmonic oscillator with SAA?