12.4 Universal Forces Objectives • Describe factors affecting gravitational forces. • Describe centripetal forces. Introduction/Review • • • • • Who developed the laws of motion? How many laws of motion exist? What is Newton’s first law of motion? What is Newton’s second law? What is Newton’s third law? Universal Forces • Forces that occur naturally everywhere in the universe are called universal forces. Universal Gravitation • Gravitational force is an attractive force that acts between any two masses. ▫ Newton’s law of universal gravitation states that every object in the universe attracts every other object. ▫ The gravitational force between any two masses is directly proportional to their masses and inversely proportional to the distance between them. Universal Gravitation Sun Earth Gravitational attraction • Gravity is the weakest of the universal forces, but is the most effective force over long distances. Universal Gravitation • The gravitational attraction between the Earth and Moon causes the ocean tides. ▫ The gravitational pull produces a bulge of water in the ocean on each side of the Earth. ▫ The rotation of the Earth beneath these bulges produces two high and two low tides each day. Centripetal Force • Centripetal force is a center-directed force that keeps a rotating object moving in a circular path. Centripetal Force Circular path Inertia Centripetal force Closure •TEST TOMORROW!!! • What will the test look like? • How many people would rather write on paper, then put their grades into the clicker? • What can you study?