# Hamiltonian

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The Hamiltonian is a quantity of great importance in both classical and quantum mechanics.

## Classical mechanics

In classical dynamics, the Hamiltonian is defined to be



where  are the generalised coordinates and  are the momenta conjugate to these coordinates, and  is the Lagrangian. For many problems the Hamiltonian is the same as the energy.

The Hamilton equations are:




### Example

For a mass  attached to a spring of spring constant  extended by a distance , . The momentum  and so





which is the familiar expression for the energy of a simple harmonic oscillator.

The equations of motion are:


.

Inserting  this into the first equation, we get . This is just Newton's second law, F = ma.

## Quantum mechanics

The Hamiltonian for many quantum mechanical systems can be obtained by writing down a corresponding classical Hamiltonian and promoting all of the coordinates and momenta to operators. The quantum mechanical Hamiltonian is of central importance to the Schrodinger equation.