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Schedule

Schedule. Today: Gravity Thursday: Light Tuesday, Feb 5: More light Thursday, Feb 7: Test 1. The Road to a Sun-Centered Solar System. Ptolemy (90-168) Almagest published ~150 Copernicus (1473-1543) Published in 1544 Tycho Brahe (1546-1601) Never published

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Schedule

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  1. Schedule • Today: Gravity • Thursday: Light • Tuesday, Feb 5: More light • Thursday, Feb 7: Test 1

  2. The Road to a Sun-Centered Solar System • Ptolemy (90-168) • Almagest published ~150 • Copernicus (1473-1543) • Published in 1544 • Tycho Brahe (1546-1601) • Never published • Johannes Kepler (1571-1630) • Published his Laws of Motion starting in 1609 • Galileo (1564-1642) • Published “Dialogue of the Two Chief World Systems” in 1632

  3. The Acceleration of Gravity (g) • Galileo showed that g is the same for all falling objects, regardless of their mass.

  4. Newton • Newton was born about a year after Galileo died • He nailed the foundation of the physics of our ordinary reality. He dominated Physics until Einstein. • He was a bit of a misanthrope. • Did poorly in school. Reports described him as 'idle' and 'inattentive'. • He went to college at Cambridge when he was 18. • Even then he was thought of as an average student.

  5. Newton • But at 20 (1663) math caught his fancy • In two years he ran through all of humankind’s mathematical knowledge and then proceeded to significantly advance it…. • The plague closed the University between 1665-1667, sending Newton back home to a bit of enforced reflection. • It was during this period that he puzzled-out most of his major contributions • Invention of calculus • Theory of gravity • The Laws of Motion • The inverse square law

  6. Newton • At 27 he became the Lucasian Professor of Mathematics at Cambridge • In later life he held a number of major positions • Member of Parliament • Master of the Mint • President of the Royal Society • The first scientist to be knighted • He was still very much a misanthrope • His response to any criticism was a bit extreme • He suffered two nervous breakdowns • He could be a pretty nasty guy. • Take his dispute with Leibniz

  7. Gravity • Take a look around you….in our everyday experience we see gravity acting on all objects.

  8. Newton made some simple observations based on his understanding of motion. • “A body remains stationary or moves in a straight line unless a force acts on it” • If a pen drops out of your hand, there HAS TO BE a force acting on that pen.

  9. Newton watched the Moon orbiting the Earth. • It is in orbit • it is moving in (essentially) a circle • Therefore…. force HAS TO BE acting on it to deflect it from a straight line. • If you know the force HAS TO EXIST, then you can calculate what its strength has to be…. • About 1/3600 of the force of gravity on the Earth’s surface. • Other fact: Moon was about 60 times farther from the CENTER OF THE EARTH than the Earth’s surface

  10. So the force on the Moon was 1/3600 as strong as the force acting on you at the surface of the Earth….. • He concluded that the force of the Earth’s gravity decreases by 1/r2. The inverse square of distance • That is how gravity (and other fundamental forces) work, their intensity decreases by the inverse square of distance. • This is called the INVERSE SQUARE LAW

  11. How does this work? Gravity • ALL matter produces a gravitation force! • Why??? • The force changes by 1/r2 • The inverse square law applies to ALL Forces….gravity, magnetism, electrical fields, light….! Light

  12. What is the difference in terms ofgravity between these pictures?

  13. Gravity • Remember gravity is measured from the center of an object • We are 6378 km from the center of the Earth • Space shuttle astronauts in space are only 200 km farther away….. • In inverse square terms that is 1.03 times

  14. Gravity • Work it out • If the distance is 1.03 times the distance at the Earth’s surface…. • R2 = 1.06 • 1/R2 x G = 9.2 m/s2 • The force of gravity is only 6% less in low Earth orbit

  15. The reason astronauts are “weightless” in orbit is because they are in FREEFALL!

  16. Remember: We live on a sphere • When you drop something it heads toward the center of the Earth • IF you put a bit of horizontal velocity on that something….

  17. Newton’s three laws of motion Newton’s first law of motion: An object moves at constant velocity unless a net force acts to change its speed or direction.

  18. Newton’s second law of motion: Force = mass  acceleration

  19. Newton’s third law of motion: For every force, there is always an equal and opposite reaction force.

  20. How do gravity and energy together explain orbits? • Orbits cannot change spontaneously. • An object’s orbit can only change if it somehow gains or loses orbital energy = kinetic energy + gravitational potential energy (due to orbit).

  21. So what can make an object gain or lose orbital energy? • Friction or atmospheric drag • A gravitational encounter. • Rockets….

  22. Center of mass • Objects attracted by gravity orbit around their center of mass

  23. Why Does Gravity Work? • Einstein’s Relativity • Quantum Gravity (String theory, Loop quantum gravity, Causal Dynamical Triangulation……and many more) • Brans–Dicke theory of gravity • Induced gravity • In the modified Newtonian dynamics (MOND) • The self-creation cosmology theory of gravity • Nonsymmetric gravitational theory (NGT) • Tensor–vector–scalar gravity (TeVeS) • Gravity as an entropic force • Superfluid vacuum theory

  24. Why are there Tides? • Take the Bay of Fundy in Nova Scotia • Tides there can range 50 feet between high and low • High (and low) tides happen basically twice a day

  25. Tidal Bores in the Severn Estuary

  26. Tides • Gravitational force decreases with (distance)2 • The Moon’s pull on Earth is strongest on the side facing the Moon, and weakest on the opposite side. • The Earth gets stretched along the Earth-Moon line. • The oceans rise relative to land at these points.

  27. Tides • Every place on Earth passes through high tides twice per day as the Earth rotates. • High tides occur every 12 hours 25minutes • remember, the Moon moves! • The Sun’s tidal effect on Earth is not as strong. Remember the inverse square law….the Sun is a long way away.

  28. Tides

  29. Spring and Neap Tides • Tides are strongest when the Sun & Moon pull in the same direction (new & full phases) • high tide is higher than usual (spring) • Weakest when the Sun & Moon pull at right angles (first & last quarter phases) • high tide is lower than usual (neap)

  30. Tidal Friction • This fight between Moon’s pull & Earth’s rotation causes friction. • Earth’s rotation slows down (1 sec every 50,000 yrs.) • Conservation of angular momentum causes the Moon to move farther away from Earth. • The Moon is moving away from Earth at of 38 mm per year!

  31. Tidal Friction • It adds up! 4 billion years ago 1 day may have only been 5 or 6 hours long. • The moon may have been 1/10 the distance: 22 Earth radii away instead of 221 • How big would the tides be if the Moon was 110 Earth radii away?

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