Assume that the earth is a solid sphere of uniform density and a tunnel is dug along its diameter throughout the earth. It is found that when a particle is released in this tunnel, it executes a simple harmonic motion. The mass of the particle is 100 g. The time period of the motion of the particle will be (approximately) 
$(\mathrm{take}\hspace{0.17em}\hspace{0.17em}\mathrm{g}=10\hspace{0.17em}{\mathrm{ms}}^{-2},\hspace{0.17em}$$\hspace{0.17em}\mathrm{radius}\mathrm{\hspace{0.17em}\hspace{0.17em}}\mathrm{of}\mathrm{\hspace{0.17em}\hspace{0.17em}}\mathrm{earth}=6400\mathrm{\hspace{0.17em}}\mathrm{km})$