# CDS 140b Spring 2014 Homework 2

R. Murray, D. MacMartin | Issued: 9 Feb 2014 (Wed) |

CDS 140b, Spring 2014 | Due: 16 Feb 2014 (Wed) |

**Note:** In the upper left hand corner of the *second* page of your homework set, please put the number of hours that you spent on
this homework set (including reading).

**UNDER CONSTRUCTION, DO NOT START**

- A planar pendulum (in the $x$-$z$ plane) of mass $m$ and length $\ell$ hangs from a support point that moves according to $x=a\cos (\omega t)$. Find the Lagrangian, the Hamiltonian, and write the first-order equations of motion for the pendulum.
**Perko, Section 3.3, problem 5.**Show that<amsmath>\aligned \dot x &=y+y(x^2+y^2)\\ \dot y &=x-x(x^2+y^2)

\endaligned</amsmath>is a Hamiltonian system with $4H(x,y)=(x^2+y^2)^2-2(x^2-y^2)$. Show that $dH/dt=0$ along solution curves of this system and therefore that solution curves of this system are given by

<amsmath> (x^2+y^2)^2-2(x^2-y^2)=C

</amsmath>Show that the origin is a saddle for this system and that $(\pm 1,0)$ are centers for this system. (Note the symmetry with respect to the $x$-axis.) Sketch the two homoclinic orbits corresponding to $C=0$ and sketch the phase portrait for this system, noting the occurrence of a compound separatrix cycle.

**Perko, Section 3.6, problem 4.**