For conservative systems (i.e. no friction or energy lose), the equation of motion can be found from the conservation of energy, which states that the sum of kinetic and potential energies is a constant,

     T + V = constant

Taking the time derivative gives

     d(T + V)/dt = 0

From this, the equation of motion for vibrations can be determined.

To understand how energy methods can be used, a simple mechanical system with linear and rotational kinetic, spring potential, and gravity potential energy will be analyzed. For the system shown, summing the kinetic and potential energies gives

Here, C is an arbitrary constant (total energy). Also, the displacement variable, y_o, needs to be related to the rotation variable, θ since only one variable can be used with the energy method.

     y_o = r θ

The moment of inertia for the pulley is

     I₁ = 1/2 m₁ r²

Substitute these equations in the total energy equation gives,

Take the first time derivative and rearrange,

Since the velocity is not zero at all times, the equation of motion is

At static equilibrium, the acceleration is zero, so the static equilibrium position is

     y_o = m₂ g/k

Rewriting the equation of motion in the new coordinate

     y = y_o - m₂ g/k

gives the familiar equation

where the natural frequency ω_n is

The general solution is

     y = A sinω_nt + B cosω_nt

The constants A and B are determined from the initial conditions: