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19.2 Gravity and the Moon p. 666-669 Gravity The force that holds the moon in orbit is a force that acts over long distances and without direct contact. Gravity is a force of attraction. Newton’s Law of universal gravitation states that gravity exists between all objects in the universe. G = 6.67 x 10-11 N m2 / kg2 Gravity is measured in Newtons. Newtons Named after Isaac Newton, it is a kg per m/s2, or a mass times an acceleration. Also a delicious cookie. Measuring Gravity 2 things tell us how strong the force of gravity is between 2 objects: 1. The masses of the objects 2. The distance between the 2 objects. Mass is the amount of matter in an object. Weight is the amount of gravity pulling on an objects and is determined by location. Oh, so you want to know the formula, do ya? The force of gravitational attraction is given by: F = G M m / r2 where F is the gravitational force (in newtons) M, m are the masses of the attracting bodies (in kilograms) r is the distance between the (centers) of the bodies G is a proportionality constant that depends on units Distance and gravity The farther away the object the less gravity’s effect. If distances increases, force decreases exponentially Inertia and orbital motion Inertia: the tendency of any object to resist a change in motion. Planets, moons, are not going to stop their motion unless something (huge) gets in their path. Newton’s first law explains inertia: an object at rest will remain at rest and an object in motion will remain in motion unless acted on by an outside force. How do Earth and moon remain in their orbits? 1. Inertia- moon keeps traveling in its path 2. Gravity- Earth keeps pulling on the moon Moon’s velocity Moon Gravitational Force Earth Path followed by the Moon Orbits The earth, moon, and the other planets are continuously falling AROUND whatever object they are orbiting. The same is true of satellites and space shuttles around earth. If you fired a rocket at a fast enough speed, it would continue to circle the earth indefinitely (8,000m/s)