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Question

Write the Hamiltonian function and find Hamilton's canonical equations for the three-dimensional motion of a projectile in a uniform gravitational field with no air resistance. Show that these equations lead to the same equations of motion as found.

Solution

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For three dimensional motion of a projectile in a uniform gravitational field with no air resistance we have:

T=m2(x˙2+y˙2+z˙2)V=mgzL=TV=m2(x˙2+y˙2+z˙2)mgz\begin{align} T &= \frac{m}{2} (\dot x^2 + \dot y^2 + \dot z^2) \\ V &= m \, g \, z \\ L &= T - V = \frac{m}{2} (\dot x^2 + \dot y^2 + \dot z^2) - m \, g \, z \end{align}

Now the generalized momentums pip_i are found using:

pi=Lq˙i\begin{equation} p_i = \frac{\partial L}{\partial \dot q_i} \end{equation}

We write:

px=Lx˙=mx˙=>x˙=pxmpy=Ly˙=my˙=>y˙=pympz=Lz˙=mz˙=>z˙=pzm\begin{align} p_x &= \frac{\partial L}{\partial \dot x} = m\dot x \quad => \dot x = \frac{p_x}{m} \\ p_y &= \frac{\partial L}{\partial \dot y} = m \dot y \quad => \dot y = \frac{p_y}{m} \\ p_z &= \frac{\partial L}{\partial \dot z} = m \dot z \quad => \dot z =\frac{p_z}{m} \end{align}

The Hamiltonian is most generally found using:

H(pi,qi)=ipiq˙i(pi,qi)L(pi,qi)\begin{equation} H(p_i, q_i) = \sum\limits_{i}p_i \dot q_i (p_i, q_i) - L(p_i, q_i) \end{equation}

but since our kinetic energy is a homogeneous quadratic function of generalized speeds (the q˙\dot q's), and our potential energy is a function of generalized coordinates (the qq's) only, we know the Hamiltonian is equal to the total energy:

H(pi,qi)=T(pi,qi)+V(pi,qi)H(pi,qi)=px22m+py22m+pz22m+mgz\begin{align} H(p_i,q_i) &= T(p_i,q_i) + V(p_i, q_i) \\ H(p_i,q_i) &= \frac{p_x^2}{2m} + \frac{p_y^2}{2m} + \frac{p_z^2}{2m} + m g z \end{align}

where we have used (5), (6), (7)

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