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Chapter 9

Chapter 9. Gravity. The Earth Sucks. 1. THE UNIVERSAL LAW OF GRAVITY. From Law 3 of Kepler, Newton deduced the inverse square law of attraction. Newton compared the fall of the moon to the falling of an apple. Moon's fall from straight line is (0.05 in/s). . m 1. m 2. r.

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Chapter 9

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  1. Chapter 9 Gravity The Earth Sucks

  2. 1. THE UNIVERSAL LAW OF GRAVITY • From Law 3 of Kepler, Newton deduced the inverse square law of attraction. • Newton compared the fall of the moon to the falling of an apple. • Moon's fall from straight line is (0.05 in/s).

  3. m1 m2 r Newton's Law of Universal Gravitation

  4. 2. The Universal Gravitational Constant, G

  5. 3. Gravity and Distance: The Inverse-Square Law • Gravity is the most dominant force in nature. • Yet it is the weakest.

  6. 4 units 3 units 2 units 1 unit Let’s look at the inverse-square nature for the force of gravity.

  7. 4. WEIGHT AND WEIGHTLESSNESS • W = mg (the gravitational force acting on m) • Weighing on an elevator and Weighing in outer space or in orbit • We define the weight of something as the force it exerts against the supporting floor or the weighing scale. (Remember the elevator video.)

  8. 5. OCEAN TIDES • demo - Earth-moon system • Force of gravity decreases with distance from moon. Moon Earth

  9. Differences in gravity on different parts of the Earth produce tidal bulges. • http://www.spacetelescope.org/videos/html/mpeg/320px/heic0705i.html North Pole High Tides

  10. Should expect two high tides, two low tides per day. • Moon moves in its orbit so that it returns to the same place in your sky every 24h 50 m. • Sun has one-half the influence of the moon • The gravitational force is greater but tides depend on the gravitation difference on opposite sides of the Earth.

  11. Sun Moon SFA

  12. First Quarter Spring Tides Sun Earth Full Moon New Moon SFA

  13. Neap Tides First Quarter Sun Earth Last Quarter SFA

  14. NASA Animation MPEG SFA

  15. Complications • Friction with ocean bottom • Irregular shape of continents • Bay of Fundi • Tides can come in very fast and exceed 15 m. SFA

  16. Tides in the Earth and Atmosphere • Tides in ionosphere produce electric currents and alter the magnetic field. • Affects penetration of cosmic rays • Can have effects on living things • Tides in the crust mean that volcanoes and earthquakes are more common at new or full moon. SFA

  17. Tides on the Moon • Just as the earth has “land tides” the moon has moon tides. • Because the moon always keeps the same face toward the earth, the moon tides are always in the same place. Go to link SFA

  18. 6. GRAVITATIONAL FIELDS • Alternate view of gravitational influence • Properties of space surrounding object are changed so that another mass entering the region experiences a force. Earth SFA

  19. Objects accelerate in the direction of the arrows. • Field is stronger where lines are closer together. Earth SFA

  20. The Gravitational Field Inside a Planet • Inside the Planet a = g a = ½g a = 0 a = ½g a = g SFA

  21. Gravitational Field • Uniform Sphere • Linear inside - like Earth outside • Hollow Sphere • Zero inside - like Earth outside SFA

  22. 7. EINSTEIN'S THEORY OF GRAVITATION Warped Space SFA

  23. Apparent position of star in this direction Sun Light from star bent by the gravity of the Sun Bending of Starlight Eye SFA

  24. 8. BLACK HOLES • Let’s observe a star that is shrinking but whose mass is remaining the same. • What happens to the force acting on an indestructible mass at the surface of the star? • What effect is there on the escape velocity? SFA

  25. F F R2 R2 F R2 F R2 which means the force is 64 times larger Remember that the force between the two masses is given by R R R R SFA

  26. If a massive star shrinks enough so that the escape velocity is equal to or greater than the speed of light, then it has become a black hole. • Particles entering it would suffer disintegration. • How are black holes detected? SFA

  27. Near a Black Hole SFA

  28. Another Look SFA

  29. 9. UNIVERSAL GRAVITATION • Earth is a "sphere" because of gravitation. • The universal law of gravitation was a great accomplishment. • Its accuracy is phenomenal. • Perturbations in planet's orbits Examples: Neptune was discovered because of Uranus's wobble. Pluto was discovered in a similar way. • Oscillating universe. Wow!

  30. Chapter 9 Review Questions

  31. If somehow the moon were moved to a distance twice as far away from the earth as it is now, the force that the earth has for the moon would (a) be twice what it is now (b) be one half of what it is now (c) be four times greater than it is now (d) be one fourth as much as it is now

  32. What must be the phase of the moon if neap tides are occurring? (a) first quarter (b) full (c) lunar eclipse (d) new

  33. Newton said that the path of a planet curves because a force acts on it. Why does a planet's path curve according to Einstein? (a) because a force acts on it (b) because it is spinning (c) because it is in a geometric warp of space and time End of Chapter Test 3 Questions End of Course

  34. Discuss final • Parting thoughts next slide

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