Let at some time particle is at a distance x from centre of Earth, then at that position fieldE=R3GM​ x∴ Acceleration of particlea=−R3GM​x⇒ω=R3GM​​=Rg​​NowT=ω2π​=2πgR​​⇒T=2×3.14×106400×103​​=2×3.14×800 sec≈1 hour 24 minutes
