What is gravity? Galileo and Newton gave the name 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.

An imaginary line from the sun sweeps out equal areas The paths of the planets are ellipses with the center of the sun at one focus. An imaginary line from the sun sweeps out equal areas in equal time intervals. Thus, planets move fastest 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 of the cubes of their average distances from the sun.

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.

F G m1 m2 = d2 In equation form: F is the force of attraction, measured in NEWTONS, between masses m1 and m2 G is the Universal Gravitation Constant, equal to 6.67 x 10-31 N m2/kg2 m1 and m2 are the attracting masses, measured in KILOGRAMS d is the distance between their centers of mass, and is measured in METERS

( ) G Me v = r 1/2 v is in meters/sec 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: ( ) 1/2 G Me v = r v is in meters/sec Me is the mass of the Earth in kilograms r is in meters G is the Universal Gravitation Constant

F g = m 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: F g = m g is the field strength, measured in Newtons/kilogram m is the test mass, measured in kilograms F is the force of attraction, measured in Newtons, on the test mass