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G r a v i t y

G r a v i t y

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G r a v i t y

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  1. G r a v i t y Chapter 9

  2. Isaac Newton (1642 - 1727)

  3. m M d Newton’s Universal Gravitation Two masses separated by a distance

  4. Newton’s Universal Gravitation

  5. Newton’s Universal Gravitation • Inverse Square Law Separation Force R F 2R ¼F 3R 1/9F ½R 4F ¼R 16F

  6. Newton’s Universal Gravitation The force of gravity cannot be made zero. • Mass causes gravity • Only one kind of mass • Contrast with the electric force • G is small • 6.67 X 10-11 N m2/kg2

  7. Apparent Weightlessness • All objects fall at the same rate in a gravitational field. • Apparent weightlessness • Lack supporting force • Examples: • Space Shuttle • Elevator • Amusement Park Rides

  8. KC-135

  9. Ocean Tides • Gravity decreases with distance Moon Earth

  10. Ocean Tides • Differences in gravity on different parts of the Earth produce tidal bulges Low Tide High Tide High Tide Low Tide

  11. Ocean Tides • 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

  12. Sun Earth Full Moon New Moon Ocean Tides • Spring Tide • Full or New Moon

  13. Ocean Tides • Neap Tide • First or Last Quarter Moon First Quarter Sun Earth Last Quarter

  14. Ocean Tides • Complications • Friction with ocean bottom • Irregular shape of continents • Bay of Fundy (Video) • Tides can come in very fast and exceed 15 m

  15. Crustal & Atmospheric Tides • 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 • Measured for the moon

  16. Earth Gravitational Field • Alternate view of gravitational influence • Properties of space surrounding object are changed so that another mass entering the region experiences a force.

  17. Gravitational Field • Objects accelerate in the direction of the arrows • Field is stronger where field lines are closer together

  18. a = g a = ½g a = ½g a = 0 a = g Gravitational Field • Inside the Planet

  19. g Distance g Distance Gravitational Field • Uniform Sphere • Linear inside • like Earth outside • Hollow Sphere • Zero inside • like Earth outside R R

  20. Space-Time No mass Distortion caused by mass

  21. General Relativity • Mass distorts space-time • Objects move in the straightest path they can in the distortions caused by mass

  22. Advance of Mercury’s Perihelion

  23. Apparent position of the star Sun Light from star bent by the gravity of the Sun Bending of Starlight

  24. Low Gravity Very small amount of bending

  25. Stronger Gravity Light at an angle is bent noticeably

  26. Exit Cone and Photon Sphere Photon Sphere

  27. Near a Black Hole

  28. Event Horizon Rs + Singularity Schwarzschild Black Hole Rs = 3(Mass) Mass Rs 3 M 9 km 5 15 10 30

  29. What Can We Know? • Mass • gravity • Charge • Electric Fields • Rotation Rate • Co-rotation

  30. Cygnus X-1

  31. Edwin Hubble

  32. Red Shift and Distance 24 Mpc 1200 km/s 300 Mpc 15,000 km/s 780 Mpc 39,000 km/s 1220 Mpc 61,000 km/s

  33. Hubble’s Result

  34. Modern Hubble’s Law

  35. 2 2 1 1 3 3 Raisin Cake Model

  36. Velocity Distance Raisin Cake Model Velocity 1 cm/hr 2 cm/hr 3 cm/hr Distance Distance from Home from Home (before baking) (afterbaking) 1 1 cm 2 cm 2 2 cm 4 cm 3 3 cm 6 cm

  37. Universal Expansion • Space-Time itself is expanding • Carries galaxies along with it • Conflicting cosmologies • Big Bang • Draw the expansion back in time until space-time occupies a point (12 - 15 billion years ago) • Steady State • Requires continual creation of matter

  38. Arno Penzias & Robert Wilson