Lecture3_Sept28_2007

Lecture3_Sept28_2007 - Early Questions, and Answers: The...

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Unformatted text preview: Early Questions, and Answers: The Rise of Physics: 1642 C.E. present The Universe is scientifically understandable, but curiouser and curiouser "All science is either physics or stamp collecting." Ernest Rutherford If at first the idea is not absurd, then there is no hope for it. Albert Einstein The Rise of Physics: 1642 C.E. present The Universe is scientifically understandable, but curiouser and curiouser Johannes Kepler (1571 C.E. - 1630 C.E.) revisited Keplers analysis of the precise observational data of Tycho led him to (reluctantly) dispose of the Aristotilean ideal of perfect circular motion in favor of elliptical motion for the planets. This is the first of Keplers Laws of Planetary Motion: 1. The orbits of planets are ellipses, with the sun at one focus of the ellipse. The other two are: 2. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse. 3. The ratio of the squares of the revolutionary periods for the two planets is equal to the ratio of the cubes of their semi major axes. The Rise of Physics: 1642 C.E. present The Universe is scientifically understandable, but curiouser and curiouser Geometrically, an ellipse (the blue line) is defined by this figure, by requiring that a+b=constant Note that the ellipticities are generally small! with the Sun at one focus planets orbit along ellipses The Rise of Physics: 1642 C.E. present The Universe is scientifically understandable, but curiouser and curiouser Keplers Second Law notes that the speed of a planet along its orbital path is not constant, but that it varies in a precise manner, such that the area swept out between the planet and the Sun is constant: equal areas in equal times. When the planet is closer to the Sun it moves faster When the planet is closer to the Sun it moves slower The Rise of Physics: 1642 C.E. present The Universe is scientifically understandable, but curiouser and curiouser Keplers Third Law relates the period of the planets orbit, P, to the size of the orbit (as given by the semi major axis of the ellipse, A). Specifically, Kepler found that P 1 2 A 1 3 ____ = ____ P 2 2 A 2 3 semi-minor axis = B semi-major axis = A The remarkable thing about Keplers Laws is that they are universal. They apply to all orbiting bodies. This suggests that there is some underlying organizing principle or physical effect: there is its gravity, as Newton showed. The Rise of Physics: 1642 C.E. present The Universe is scientifically understandable, but curiouser and curiouser Isaac Newton (1642 C.E. - 1727 C.E.) Newton is considered by many to be the most influential scientist ever. He had a profound influence on science and physics; his landmark work Philosophea Naturalis Principia Mathematica laid the ground work for a physical undersanding of the Solar System and gravitation. In the Principia Newton developed three Laws of Motion: 1. Bodies at rest stay at rest until acted on by a force. Bodies in uniform motion1....
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Lecture3_Sept28_2007 - Early Questions, and Answers: The...

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