
Two objects which have equal masses, m, are connected with a spring whose force constant is k. The objects are placed on a frictionless surface and oscillating without external forces. Find the natural frequency of this motion.
Answer:
x_1 and x_2 are the displacements of each object from the natural length. The displacement, x, is the one for the spring, so x=x_1-x_2 for object 1 and -x=x_2-x_1 for object 2. Set up the equations of motion:
\begin{eqnarray} mx_1'' &=& -kx \\ mx_2'' &=& kx \end{eqnarray}
Subtract (2) from (1).
\begin{equation} m(x_1''-x_2'') = -2kx \end{equation}
Since x'' = x_1''-x_2'', we have
\begin{equation} mx'' + 2kx = 0 \quad \rightarrow \quad x'' + \frac{2k}{m} = 0 \end{equation}
The solution of this differential equation is
\begin{equation} x = A\sin\left(\sqrt{\frac{2k}{m}} t\right) + B\cos\left(\sqrt{\frac{2k}{m}} t\right) \end{equation}
where \sqrt{\frac{2k}{m}} is the natural frequency of this spring motion.
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