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gravity force mass weight acceleration physics natural phenomenon universal law Earth pull attraction gravitational field motion science orbit planetary force energy density free fall equilibrium Newton theory space-time curvature gravitational pull terrestrial force celestial bodies physics concept downward force measurement universal attraction physics principle mass interaction gravitational energy force of nature gravity effect falling objects gravity experiment weight measurement gravity constant gravitational acceleration planetary motion gravitational physics astrophysics mechanics gravity simulation Earth gravity force field scientific study gravity influence motion law gravitational system universal force theoretical physics physical science gravitational interaction gravity theory planetary gravity environment gravity strength Earth science gravitational pull effect