Sir Isaac Newton was the first to postulate gravity as we know it; a force exerted by anything with mass on anything else with mass, according to the product of the masses used and inversely...
At or near the surface of the earth, it's 9.8 meters (32.2 feet) per second2 . It's different at significant altitudes above the earth's surface, or on the surface of other, extraterrestrial bodies.
ag = GM/d2.ag is the gravitational acceleration; G is the gravitational constant; M is the mass of the primary body (e.g. planet); d is the distance between the centres of mass of the primary and...