= Uniform-Earth chord oscillator
{title2=$\omega_0^2=4\pi G\rho/3$}
Inside a homogeneous spherical <mass>, <gravity> is proportional to position. Projection onto a fixed straight chord gives $\ddot y=-\omega_0^2y$, independently of its perpendicular offset from the <sphere> centre. Release from one surface endpoint at rest reaches the opposite endpoint after $\pi/\omega_0$. The transverse <force> is supplied by the tunnel constraint. This idealization neglects removed tunnel <mass>, rotation, density variation and dissipative <forces>.
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