l5 - Today Announcements HW#3 is due by 8:00 am Wednesday...

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ISP209s8 Lecture 5 -1- Today • Announcements: – HW#3 is due by 8:00 am Wednesday January 30th. – A second extra credit problem will be available next week • Gravity •F o r c e • Vectors
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ISP209s8 Lecture 5 -2- History of astronomy • Ptolomy devised an Earth-centered model of the motion of planets that worked well • Brahe made detailed measurements that showed deficiencies in the model • Kepler discarded the previous assumptions and devised his three laws of planetary motion with the Sun at the center • Newton unified the tree laws with his Universal Law of Gravity
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ISP209s8 Lecture 5 -3- Planetary Orbits • We will use the program Interactive Physics to demonstrate planetary orbits. • Newton recognized the acceleration of the Moon and knew that a force from the Earth must be responsible. He called this gravity. • At the time it was questioned because people would not accept force at a distance. • But, what does cause gravity? More later …
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ISP209s8 Lecture 5 -4- Newton’s Universal Law of Gravity Newton’s Universal Law of Gravity: 2 2 2 2 1 11 673 . 6 ; kg Nm E G r m Gm F = = WHY?
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ISP209s8 Lecture 5 -5- The Law of Gravity What is the force of gravity on a 90 kg professor standing on the surface of the Earth? () N m E kg E kg kg Nm E r m Gm F e e p 882 6 378 . 6 24 974 . 5 90 11 673 . 6 2 2 2 2 = × = = Note: We get the same answer if we ask what is the force a professor exerts on the Earth. That is Newton’s third law.
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l5 - Today Announcements HW#3 is due by 8:00 am Wednesday...

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