14.E: Coupled linear oscillators (Exercises)
- Page ID
- 14261
1. Two particles, each with mass \(m\), move in one dimension in a region near a local minimum of the potential energy where the potential energy is approximately given by \[U = \frac{1}{2} k (7x^2_1 + 4x^2_2 + 4x_1x_2)\nonumber\] where \(k\) is a constant.
- Determine the frequencies of oscillation.
- Determine the normal coordinates.
2. What is degeneracy? When does it arise?
3. The Lagrangian of three coupled oscillators is given by: \[\sum^3_{n=1} \left[\frac{m\dot{x}^2_n}{2} - \frac{kx^2_n}{2} \right] + k^{\prime}(x_1x_2+x_2x_3).\nonumber\] Find \(x_2(t)\) for the following initial conditions (at \(t = 0\)): \[(x_1, x_2, x_3)=(x_0, 0, 0), :: (\dot{x}_1, \dot{x}_2, \dot{x}_3) = (0, 0, v_0). \nonumber\]
4. A mechanical analog of the benzene molecule comprises a discrete lattice chain of 6 point masses \(M\) connected in a plane hexagonal ring by 6 identical springs each with spring constant \(\kappa\) and length \(d\).
- List the wave numbers of the allowed undamped longitudinal standing waves.
- Calculate the phase velocity and group velocity for longitudinal travelling waves on the ring.
- Determine the time dependence of a longitudinal standing wave for a angular frequency \(\omega = 2\omega_{cutoff}\), that is, twice the cut-off frequency.
5. Consider a one dimensional, two-mass, three-spring system governed by the matrix \(A\), \[A = \begin{pmatrix} 4 & -2 \\ -2 & 7 \end{pmatrix}\nonumber\] such that \(Ax = \omega^2x\),
- Determine the eigenfrequencies and normal coordinates.
- Choose a set of initial conditions such that the system oscillates at its highest eigenfrequency.
- Determine the solutions \(x_1(t)\) and \(x_2(t)\).
6. Four identical masses \(m\) are connected by four identical springs, spring constant \(\kappa\), and constrained to move on a frictionless circle of radius \(b\) as shown on the left in the figure.
- How many normal modes of small oscillation are there?
- What are the eigenfrequencies of the small oscillations?
- Describe the motion of the four masses for each eigenfrequency.
7. Consider the two identical coupled oscillators given on the right in the figure assuming \(\kappa_1 = \kappa_2 = \kappa\). Let both oscillators be linearly damped with a damping constant \(\beta\). A force \(F = F_0 \cos(\omega t)\) is applied to mass \(m_1\). Write down the pair of coupled differential equations that describe the motion. Obtain a solution by expressing the differential equations in terms of the normal coordinates. Show that the normal coordinates \(\eta_1\) and \(\eta_2\) exhibit resonance peaks at the characteristic frequencies \(\omega_1\) and \(\omega_2\) respectively.
8. As shown on the left below the mass \(M\) moves horizontally along a frictionless rail. A pendulum is hung from \(M\) with a weightless rod of length \(b\) with a mass \(m\) at its end.
- Prove that the eigenfrequencies are \[\omega_1 = 0 \quad \omega_2 = \sqrt{\frac{g}{Mb} (M + m)} \nonumber\]
- Describe the normal modes.