2. What is gravity? GalileoGalileo and Newton gave the name Newton GalileoNewton gravity to the force that exists between the Earth and objects. Newton showed that the same force exists between all objects. Even today, no one knows exactly why objects attract each other.

3.  The paths of the planets are ellipses with the center of the sun at one focus. of the sun at one focus.  An imaginary line from the sun sweeps out equal areas in equal time intervals. Thus, planets move fastest in equal time intervals. Thus, planets move fastest when closest to the sun, slowest when farthest away. when closest to the sun, slowest when farthest away.  The ratio of the squares of the periods of any two planets revolving about the sun is equal to the ratio planets revolving about the sun is equal to the ratio of the cubes of their average distances from the sun. of the cubes of their average distances from the sun. link1link1, link2, link3 link2link3 link1link2link3

4. In 1666, Isaac Newton determined that the same force that kept the planets in motion must also exist between every object. He stated that every object in the universe attracts every other object in the universe. The force of attraction between two objects is directly proportional to the product of their masses, and inversely proportional to the square of their separation distance. link1link1, link2, link3 link2link3 link1link2link3

5. In equation form: F  F is the force of attraction, measured in NEWTONS, between masses m 1 and m 2 G G is the Universal Gravitation Constant, equal to 6.67 x 10 -31 N m 2 /kg 2 m1m1 m2m2 m 1 and m 2 are the attracting masses, measured in KILOGRAMS d2d2 d is the distance between their centers of mass, and is measured in METERS

6. The Earth’s gravitational attraction provides the centripetal force needed to keep a satellite in orbit. The linear velocity v needed to keep a satellite in a circular orbit a distance r above the Earth’s surface is given by: v = v is in meters/sec G G is the Universal Gravitation Constant MeMe M e is the mass of the Earth in kilograms r () r is in meters 1/2

7. A gravitational field exists around every object that has mass. The gravitational field always points directly toward the object’s center of mass. The magnitude of the field is given by: g g is the field strength, measured in Newtons/kilogram = F F is the force of attraction, measured in Newtons, on the test mass m m is the test mass, measured in kilograms