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Newton wondered about the force that must act to keep the Moon in its circular orbit around Earth. Newton realized that since falling objects accelerate Whenever a body has a force exerted on it, that force is exerted by some other body. But what exerts the force of gravity? Every object on Earth feels gravity directed toward the center of Earth. Newton concluded that Earth itself exerts the gravitational force on objects at its surface Newton’s “Apple Story” led him to think about how gravity acts on the top of trees, and mountains, etc. then maybe it could act all the way to the moon! There was much controversy at the time because people were so used to the idea of contact forces only. If gravity acts between planets, their moons, and the sun, then is it possible gravity acts between all objects? Is there a gravitation force between you and your classmates? YES! Law of Universal Gravitation - Every particle in the universe attracts every other particle in the universe - F = G m1m2 / r2 - This force acts along the line joining the two particles G= 6.67 x 10-11 Nm2/kg2 (Universal Constant) When this equation is applied to the gravitational force between Earth and an object at its surface then the equation becomes: g = G mE / rE2 radius of Earth = 6.38 x 106 m mass of Earth = 5.98 x 1024 kg *Until the value of G was determined in 1798 the mass of the Earth was unknown! Examples: A 50 kg person and a 75 kg person are sitting on a bench about 50 cm apart. Estimate the magnitude of the gravitational force each exerts on the other. What is the acceleration due to gravity on the top of Mt. Everest, which is 8,848 m above the Earth’s surface?