When the particle is outside the tunnel force acting on it is \(\propto \frac{1}{r^2}\) where, \(r\) is the distance from the centre of the earth.
When the particle is inside the tunnel force acting on it is \(\propto \frac{1}{r}\) where, \(r\) is the distance from the centre of the earth.
In both cases, force is always directed towards the centre of the earth.
Thus motion is oscillatory and also periodic but not \(SHM\).
$y=A \sin (\pi t+\phi)$
લોલકની લંબાઈ ..........$cm$