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# Earth – moon gravity primer

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### Transcript

• 1. Earth – Moon Gravity Primer 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. Newton's Law of Universal Gravitation r m 1 m 2
• 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. Let’s look at the inverse-square nature for the force of gravity. 1 unit 2 units 3 units 4 units
• 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
High Tides North Pole
• 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 24 h 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. Sun Earth Spring Tides SFA New Moon Full Moon First Quarter
• 13. Sun Earth First Quarter Last Quarter Neap Tides 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.
• 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 = g a = 0 a = ½g a = ½g 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. Bending of Starlight Eye SFA Sun Light from star bent by the gravity of the Sun Apparent position of star in this direction
• 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. R R R R Remember that the force between the two masses is given by SFA R 2 F R 2 F R 2 F R 2 F which means the force is 64 times larger
• 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 &quot;sphere&quot; 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 Course End of Chapter Test 3 Questions
• 34.
• Discuss final
• Parting thoughts next slide