 Solution

ID: past-exam-of-the-theoretical-physics-i-course-of-the-university-of-cambridge/2013/paper-1/3/a/solution

{f, g} = i \sum (\frac{\partial f}{\partial q _{i}} \frac{\partial g}{\partial p _{i}} - \frac{\partial f}{\partial p _{i}} \frac{\partial g}{\partial q _{i}}) .

In particular,

{q_{j}, g} = \partial_{p_{j}} g

{p_{j}, g} = - \partial_{q_{j}} g

{f, q_{j}} = - \partial_{p_{j}} f

and

{f, p_{j}} = \partial_{q_{j}} f

. Hence

{q_{i}, p_{j}} = δ_{ij}

and the coordinate–coordinate and momentum–momentum brackets vanish.

\frac{df}{d t} = \partial_{t} f + \sum_{i} (\partial_{q_{i}} f \overset{q}{˙}_{i} + \partial_{p_{i}} f \overset{p}{˙}_{i}) = \partial_{t} f + {f, H} .

D_{t} = \partial_{t} + {\cdot, H}

D_{t} {f, g} = {D_{t} f, g} + {f, D_{t} g} .

To see the Hamiltonian part explicitly, Jacobi rearranges to

{{f, g}, H} = {{f, H}, g} + {f, {g, H}}

. Therefore

D_{t} f = D_{t} g = 0

implies $D_{t} {f, g} = 0$ , even when the conserved functions depend explicitly on time.

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