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Page 89-90 - What is This Law of Gravitation?
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Quotes "[W]e should take some time off to stand in awe of a nature that could follow with such completeness and generality such an elegantly simple principle as the law of gravitation."
"This was a tremendous idea - that to find something out, it is better to perform some careful experiments than to carry on deep philosophical arguments." |
Questions: What is missing from this equation? Or it is? - shape of the matter (spherical, cubic, oblong, rough?) - type of matter (gold, silver, lead, oxygen?) - state of matter (gas, liquid, solid?) - temperature of matter?
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Key terms: Gravitational Constant "G" acceleration Test Yourself! The Gravity Quiz! |
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Pages 90-92- Kepler's Laws
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Quotes: "First of all, Kepler found that each planet goes around the sun in a curve called an ellipse, with the sun at a focus of the ellipse." "Kepler's second observation was that the planets do not go around the sun at a uniform speed, but move faster when they are nearer the sun and more slowly when they are farther away from the sun..." "Finally, a third law was discovered by Kepler much later; this law is of a different category form the other two, because it deals not with only a single planet, but relates one planet to another. This law says that when the orbital period and orbit size of any two planets are compared, the periods are proportional to the 3/2 power of the orbit size."
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Questions: How can you restate Kepler's Laws in common words? - SHAPE - SPEED - TIME
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Check out:
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Page 92-93 Dynamics
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Quotes " Galileo discovered a very remarkable fact about motion, which was essential for understanding these laws. That is the principle of inertia - if something is moving, with nothing touching it and completely undisturbed, it will go on forever, coasting at a uniform speed in a straight line." "Newton thus added the idea that a force is needed to change the speed or the direction of motion of a body." "In other words, because of the principle of inertia, the force needed to control the motion of a planet around the sun is not a force around the sun but toward the sun."
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Questions: |
Key terms: Force - the only way modify the motion of a body with mass, either in speed or direction (or both.)
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Pages 94 - 96 - Newton's Law of Gravitational Force
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Quotes "...Newton felt certain that each planet held its moons with a force. He already knew of the force holding us on the earth, so he proposed that this was a universal force - that everything pulls everything else." "Six years later a new measurement of the size of the earth showed that the astronomers had been using an incorrect distance to the moon. When Newton heard of this, he made the calculation [of the distance the moon should "fall" each second towards Earth to account for its orbital motion] again, with the corrected figures, and obtained beautiful agreement."
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Pages 97-98 - What does the law of gravity explain?
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Quotes: "Any great discovery of a new law is useful only if we can take more out than we put in." "The law of gravitation explains many phenomena not previously understood. For example, the pull of the moon on the earth causes the tides." |
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Pages 80-81 Kinetic Energy
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Quotes: "[W]e can easily see that in the motion at the bottom must be a quantity of energy which permits [the pendulum mass] to rise to a certain height, and which has nothing to do with the machinery by which it comes up or the path by which it comes up." "Kinetic Energy = [weight x (velocity)^2] / 2g"
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Questions: Why does Feynman say that the formulae for gravitational potential and kinetic energies are incorrect? |
Key terms: "g" in the formula for kinetic energy is the earth's gravitational constant of acceleration at the surface of the planet = 9.8 meters/s/s, about 32 feet per sec of velocity change/per sec
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Pages 98 -104 Universal Gravitation
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Quotes: "What else can we understand when we understand gravity?... We can thus deduce that the sun, the moon, and the earth should be (nearly) spheres, just from the law of gravitation." "This [discrepancy in the observed schedule of Jupiter's moons being eclipsed by the planet] would have been a very difficult thing to explain according to the law of gravitation - it would have been, in fact, the death of this wonderful theory if there were no other explanation." - discoveries of other planets (Neptune, Pluto) - discoveries of double stars (Mizar) orbiting a common center of gravity. Also see: A computed model for a binary star system. - Galaxies |
Questions:
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Key terms:
centrifugal effects = the apparent outward motion (or force) of objects in rotation, really due to their own inertia taking them in a straight line path while they are being forced to move in a circle. To be "thrown" outward in a car seat when cornering is not really due to an outward force, but to the tendency of remaining moving along a straight-line path.
Olaus (Ole) Roemer, who studied the advance and retard of observing Jupiter's moons and reasoned from this a finite speed of light.
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Pages 107-110 What is Gravity?
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Quotes: "It is characteristic of the physical laws that they have this abstract character." "Many mechanisms for gravitation have been suggested. It is interesting to consider one of these, which many people have thought of from time to time." "No machinery has ever been invented that "explains" gravity without also predicting some other phenomenon that does not exist."
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Questions:
Can gravity sing? Check out: "The best popular science essay ever" by Keith Devlin.
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Key Terms:
Lagrangian points = a point between the Earth and Sun where the orbit of a satellite may be maintained synchronously with Earth because Earth's gravity offsets a portion of the Sun's gravitational pull.
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Page 110-113 Gravitational Change? Relativity?
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Quotes: " [S]o it has been proposed that the gravitational constant is related to the age of the universe. If that were the case, the gravitational constant would change with time, because as the universe got older the ratio of the age of the universe to the time which it takes for light to go across a proton would be gradually increasing. Is it possible that the gravitational constant is changing with time?" "Einstein advanced arguments which suggest that we cannot send signals faster than the speed of light, so the law of gravitation must be wrong." |
Questions: How does Feynman propose to answer the question about whether gravity has changed over time? See: Are the Laws of Nature changing with Time?
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Key terms:
Check out current results in gravitation relativity.
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