Sunday, March 2, 2014

Hamiltonian formalism exercises

Exercise 1 Choose a set of generalized coordinates that totally specify the mechanical state of the following systems:

  1. A particle of mass $ {m}$ that moves along an ellipse. Let $ {x=a\cos\theta}$ and $ {y=b\sin\theta}$. Then the generalized coordinate is $ {\theta}$. Perhaps you might be surprised to find out that we need only a single coordinate to describe the motion of this particle since its motion is described on a plane which is a two dimensional entity. But the particle motion is restricted to be along the ellipse and that constraint decreases the particle's degrees of freedom to one instead of being two.
  2. A cylinder that moves along an inclined plane. If the cylinder rotates we need $ {x}$, the distance travelled, and $ {\theta}$, the angle of rotation. If the cylinder doesn't rotate we only need $ {x}$.
  3. The two masses on a double pendulum.
    The generalized coordinates are $ {\theta_1}$ and $ {\theta_2}$.
    Do you see why? 

Exercise 2 Derive the transformation equations for the double pendulum.
It is $ {x_1=l_1\cos\theta_1}$, $ {x_2=l_1\cos\theta_1+l_2\cos\theta_2}$, $ {y_1=l_1\sin\theta_1}$ and $ {y_2=l_1\sin\theta_1+l_2\sin\theta_2}$

Exercise 3 Show that $ {\dfrac{\partial \dot{\vec{r}}_\nu}{\partial \dot{q}_\alpha}=\dfrac{\partial\vec{r}_\nu}{\partial q_\alpha}}$.
$ {\begin{aligned} \vec{r}_\nu&=\vec{r}_\nu(q_1,q_2,\cdots,q_n,t)\Rightarrow \\ \dot{\vec{r}_\nu}&=\dfrac{\partial\vec{r}_\nu }{\partial q_1}\dot{q}_1+\cdots+\dfrac{\partial\vec{r}_\nu }{\partial q_n}\dot{q}_n+\dfrac{\partial\vec{r}_\nu}{\partial t} \Rightarrow \\ \dfrac{\partial \dot{\vec{r}}_\nu}{\partial\dot{q}_\alpha}&=\dfrac{\partial \vec{r}_\nu}{\partial q_\alpha} \end{aligned}}$

Exercise 4 Consider a system of particles that experiences an increment $ {dq_j}$ on its generalized coordinates. Derive the following expression $ {dW=\displaystyle \sum_\alpha \Phi_\alpha dq_\alpha}$ for the total work done by the force and indicate the physical meaning of $ {\Phi_\alpha}$.
First note that

$ \displaystyle d\vec{r}_\nu =\sum_{\alpha=1}^n \dfrac{\partial \vec{r}_\nu}{\partial q_\alpha}dq_\alpha$
For $ {dW}$ it is
$ {\begin{aligned} dW &=\sum_{\nu=1}^N\vec{F}_\nu\cdot d\vec{r}_\nu\\ &=\sum_{\nu=1}^N\left( \sum_{\alpha=1}^n \vec{F}_\nu\cdot\dfrac{\partial \vec{r}_\nu}{\partial q_\alpha} \right) dq_\alpha\\ &= \sum_{\alpha=1}^n \Phi_\alpha dq_\alpha \end{aligned}}$
with $ {\displaystyle \Phi_\alpha=\sum_{\nu=1}^N \vec{F}_\nu\cdot\dfrac{\partial \vec{r}_\nu}{\partial q_\alpha}}$ being the generalized force.

Exercise 5 Show that $ {\Phi_\alpha=\dfrac{\partial W}{\partial q_\alpha}}$.
We have $ {\displaystyle dW=\sum_\alpha\frac{\partial W}{\partial q_\alpha}dq_\alpha}$ and $ {\displaystyle dW=\sum_\alpha\Phi_\alpha dq_\alpha}$. Hence $ {\displaystyle \sum_\alpha\left( \Phi_\alpha- \frac{\partial W}{\partial q_\alpha}\right)dq_\alpha=0}$. Since $ {dq_\alpha}$ are linearly independent it is $ {\Phi_\alpha=\dfrac{\partial W}{\partial q_\alpha}}$.

Exercise 6 Derive the lagrangian for a simple pendulum and obtain an equation to describe its motion.
The generalized coordinate for the simple pendulum is $ {\theta}$ and the transformation equations are $ {x=l\sin\theta}$ and $ {y=-l\cos\theta}$.
For the kinetic energy it is $ {K=1/2mv^2=1/2m(l\dot{\theta})^2=1/2ml^2\dot{\theta}^2}$.
for the potential $ {V=mgl(1-\cos\theta)}$.
Hence the Lagrangian is $ {L=K-V=1/2ml^2\dot{\theta}^2-mgl(1-\cos\theta)}$.
$ {\dfrac{\partial L}{\partial \theta}=-mgl\sin\theta}$ and $ {\dfrac{\partial L}{\partial \dot{\theta}}=ml^2\dot{\theta}}$.
Hence the Euler Lagrange equation is
$ {\begin{aligned} \frac{d}{dt}\dfrac{\partial L}{\partial \dot{\theta}}-\dfrac{\partial L}{\partial \theta}&=0\Rightarrow\\ ml^2\dot{\theta}+mgl\sin\theta&=0\Rightarrow\\ \ddot{\theta}+g/l\sin\theta=0 \end{aligned}}$

Exercise 7 Two particles of mass $ {m}$ are connected with each other and to two points $ {A}$ and $ {B}$ by springs with constant factor $ {k}$. The particles are free to slide along the direction of $ {A}$ and $ {B}$. Use the Euler-Lagrange equations to derive the equations of motion of the particles.
The kinetic energy is $ {K=1/2m\dot{x}^2_1+1/2m\dot{x}^2_2}$.
The potential energy is $ {V=1/2kx^2_1+1/2k(x_2-x_1)^2+1/2kx^2_2}$.
Hence the Lagrangian is $ {L=1/2m\dot{x}^2_1+1/2m\dot{x}^2_2-1/2kx^2_1-1/2k(x_2-x_1)^2-1/2kx^2_2}$.
The partial derivatives of the Lagrangian are:

  1. $ {\dfrac{\partial L}{\partial x_1}=k(x_2-x_1)}$
  2. $ {\dfrac{\partial L}{\partial x_2}=k(x_1-x_2)}$
  3. $ {\dfrac{\partial L}{\partial \dot{x_1}}=m\dot{x}_1}$
  4. $ {\dfrac{\partial L}{\partial \dot{x_2}}=m\dot{x}_2}$
And the Euler-Lagrange equations are:

  1. $ {m\ddot{x}_1=k(x_2-x_1)}$
  2. $ {m\ddot{x}_2=k(x_1-2x_2)}$

Exercise 8 A particle of mass $ {m}$ moves subject to a conservative force field. Use cylindrical coordinates to derive:

  1. The Lagrangian. The kinetic energy is $ {K=1/2m(1/2m\dot{\rho}^2+\rho^2\dot{\phi}^2+\dot{z^2})}$. The potential is $ {V=V(\rho,\phi,z)}$.
  2. The equations of motion.
    • $ {m(\ddot{\rho}-\rho\dot{\phi}^2)=-\dfrac{\partial v}{\partial \rho}}$
    • $ {m\dfrac{d}{dt}(\rho^2\dot{\phi}=-\dfrac{\partial V}{\partial \phi}}$
    • $ {m\ddot{z}=-\dfrac{\partial V}{\partial z}}$

Exercise 9 A double pendulum oscillates on a vertical plane.


  1. The Lagrangian. The transformation equations for the coordinates are
    • $ {x_1=l_1\cos\theta_1}$
    • $ {y_1=l_1\sin\theta_1}$
    • $ {x_2=l_1\cos\theta_1+l_2\cos\theta_2}$
    • $ {y_2=l_1\sin\theta_1+l_2\sin\theta_2}$
    Applying $ {\dfrac{d}{dt}}$ to the previous equations

    • $ {\dot{x}_1=-l_1\dot{\theta}_1\sin\theta_1}$
    • $ {\dot{y}_1=l_1\dot{\theta}_1\cos\theta_1}$
    • $ {\dot{x}_2=-l_1\dot{\theta}_1\sin\theta_1-l_2\dot{\theta}_2\sin\theta_2}$
    • $ {\dot{y}_2=l_1\dot{\theta}_1\cos\theta_1+l_2\dot{\theta}_2\cos\theta_2}$
    Hence the kinetic energy is

    $ \displaystyle K=1/2m_1l^2_1\theta^2_1+1/2m\left[ l^2_1\dot{\theta}^2_1+l^2_2\dot{\theta}^2_2+2l_1l_2\dot{\theta}_1\dot{\theta}_2\cos(\theta_1-\theta_2) \right]$
    And the potential is

    $ \begin{aligned}\displaystyle V&=m_1g(l_1+l_2-l_1\cos\theta_1)\\ &+m_2g\left[l_1+l_2-(l_1\cos\theta_1+l_2\cos\theta_2)\right]\end{aligned}$
    As always the Lagrangian is $ {L=K-V=\cdots}$
  2. The equations of motion.
    • ${\begin{aligned}\dfrac{\partial L}{\partial \theta_1}&=-m_2l_1l_2\dot{\theta}_1\dot{\theta}_2\sin(\theta_1-\theta_2)\\ &- m_1gl_1\sin\theta_1-m_2gl_1\sin\theta_1\end{aligned}}$
    • $ {\dfrac{\partial L}{\partial \dot{\theta_1}}=m_1 l_1^2\dot{\theta}_1^2+m_2l_1^2\dot{\theta}_1+m_2l_1l_2\dot{\theta}_2\cos(\theta_1-\theta_2)}$
    • $ {\dfrac{\partial L}{\partial \theta_2}=m_2l_1l_2\dot{\theta}_1\dot{\theta}_2\sin(\theta_1-\theta_2)-m_2gl_2\sin\theta_2}$
    • $ {\dfrac{\partial L}{\partial \dot{\theta_2}}=m_2l_2^2\dot{\theta}_2^2+m_2l_1l_2\dot{\theta}_1\cos(\theta_1-\theta_2)}$
    ${\begin{aligned}-(m_1+m_2)gl\sin\theta_1&=(m_1+m_2)l_1\ddot{\theta}_1+ m_2l_1l_2\ddot{_2}\cos(\theta_1-\theta_2)\\ &+ m_2l_1l_2\dot{\theta}_2\sin(\theta_1-\theta_2)\end{aligned}}$
    $ {\begin{aligned}-m_2gl_2\sin\theta_2 &=m_2l_2\ddot{\theta}_2+m_2l_1l_2\ddot{\theta}_1\cos(\theta_1-\theta_2)\\ &+ m_2l_1l_2\dot{\theta}_1^2\sin(\theta_1-\theta_2)\end{aligned}}$
  3. Assume that $ {m_1=m_2=m}$ e $ {l_1=l_2=l}$ and write the equations of motion. Left as an exercise for the reader.
  4. Write the previous equations in the limit of small oscillations. If $ {\theta\ll1}$ implies $ {\sin \theta\approx\theta}$ and $ {\cos \theta\approx1}$.
    Hence the equations of motion are
    $ {2l\ddot{\theta}_1+l\ddot{\theta}_2=-2g\theta_1}$
    $ {l\ddot{\theta}_1+l\ddot{\theta}_2=-g\theta_2}$ 

Exercise 10 A particle moves along the plane $ {xy}$ subject to a central force that is a function of the distance between the particle and the origin.

  1. Find the hamiltonian of the system. The generalized coordinates are $ {r}$ and $ {\theta}$. The potential is of the form $ {V=V(r)}$.
    The Lagrangian is $ {L=1/2m(\dot{r}^2+r^2\dot{\theta}^2)-V(r)}$.
    The conjugate momenta are:

    • $ {p_r=\dfrac{\partial L}{\partial \dot{r}}=m\dot{r}}$
    • $ {p_\theta=\dfrac{\partial L}{\partial \dot{\theta}}=mr^2\dot{\theta}}$
    For the Hamiltonian it is
    $ {\begin{aligned} H&=\displaystyle \sum_{\alpha=n}^np_\alpha\dot{q}_\alpha\\ &=p_r\dot{r}+p_\theta\dot{\theta}-(1/2m(\dot{r^2}+r^2\dot{\theta}^2)-V(r))\\ &=\dfrac{p_r^2}{2m}+\dfrac{p_theta^2}{2mr^2}+V(r) \end{aligned}}$
  2. Write the equations of motion.
    • $ {\dot{r}=\dfrac{\partial H}{\partial p_r}=\dfrac{p_r}{m}}$
    • $ {\dot{\theta}=\dfrac{\partial H}{\partial p_\theta}=\dfrac{p_\theta}{mr^2}}$
    • $ {\dot{p}_r=-\dfrac{\partial H}{\partial r}=\dfrac{p_\theta}{mr^3}-V'(r)}$
    • $ {\dot{p}_\theta=-\dfrac{\partial H}{\partial \theta}=0)}$

Exercise 11 A particle describes a one dimensional motion subject to a force

$ \displaystyle F(x,t)= \frac{k}{x^2}e^{-t/\tau} $
where$ {k}$ and $ {\tau}$ are positive constants. Find the lagrangian and the hamiltonian.
Compare the hamiltonian with the total energy and discuss energy conservation for this system.
Since $ {F(x,t)= \frac{k}{x^2}e^{-t/\tau}}$ it follows $ {V=\dfrac{k}{x}e^{-t/\tau}}$.
For the kinetic energy it is $ {K=1/2m\dot{x}^2}$. Hence the lagrangian is

$ \displaystyle L=1/2m\dot{x}^2-\dfrac{k}{x}e^{-t/\tau}$
. Now $ {p_x=m\dot{x}\Rightarrow\dot{x}=\dfrac{p_x}{m}}$.
For the Hamiltonian it is $ {H=p_x\dot{x}-L=\dfrac{p_x^2}{2m}+\dfrac{k}{x}e^{-t/\tau}}$.
Since $ {\dfrac{\partial L}{\partial t}=0}$ the system isn't conservative. Since $ {\dfrac{\partial U}{\partial \dot{x}}=0}$ it is $ {H=E}$.

Exercise 12 Consider two functions of the generalized coordinates and the generalized momenta, $ {g(q_k,p_k)}$ and $ {h(q_k,p_k)}$. The Poisson brackets are defined as:

$ \displaystyle [g,h]=\sum_k \left(\frac{\partial g}{\partial q_k}\frac{\partial h}{\partial p_k}-\frac{\partial g}{\partial p_k}\frac{\partial h}{\partial q_k}\right) $
Show the following properties of the Poisson brackets:

  1. $ {\dfrac{dg}{dt}=[g,H]+\dfrac{\partial g}{\partial t}}$. Left as an exercise for the reader.
  2. $ {\dot{q}_j=[q_j,H]}$ e $ {\dot{p}_j=[p_j,H]}$. Left as an exercise for the reader.
  3. $ {[p_k,p_j]=0}$ e $ {[q_k,q_j]=0}$. Left as an exercise for the reader.
  4. $ {[q_k,p_j]=\delta_{ij}}$. Left as an exercise for the reader.
    If the Poisson brackets between two functions is null the two functions are said to commute.
    Show that if a function $ {f}$ doesn't depend explicitly on time and $ {[f,H]=0}$ the function is a constant of movement. 


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