A short, straight and frictionless tunnel is bored through the centre of the earth and a body is released from the surface into the tunnel. Show that the motion of the body in the tunnel will be simple harmonic and hence calculate the time taken by the body to travel from one end of the tunnel to the other. (Radius of the earth = 6.38 x 106 m and acceleration due to gravity at the surface = 9.81 m s-2 )