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Chapter 7

Chapter 7. Gravity. 7.1 Legend of Falling Apple. Newton was your age, sat under tree, saw apple fall Looked at the moon Gravity holds apple, moon to Earth. 7.2 Fact of Falling Moon. Why doesn’t moon fall towards Earth?

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Chapter 7

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  1. Chapter 7 Gravity

  2. 7.1 Legend of Falling Apple • Newton was your age, sat under tree, saw apple fall • Looked at the moon • Gravity holds apple, moon to Earth

  3. 7.2 Fact of Falling Moon • Why doesn’t moon fall towards Earth? • Depends on how you see the path (think straight drop vs. falling in motion-curved) • Moon has tangential velocity- parallel to Earth’s surface • Moon falls around Earth • If velocity = 0, BOOM!

  4. 7.3 Law of Universal Gravitation • Newton- gravity not only act on things, but between things (your car at rest, other cars speed by, pull your car) • Law of universal gravitation- force is inversely proportional to the square of the distance separating them • F~ m1m2/d^2 • See examples on board

  5. 7.4 Gravity and Distance • Farther you get, gravity is weaker • Examples • As you spread out, brightness decreases or power to affect decreases • 3 times as far, feel 1/9 • 2 times as far, feel ¼ • 5 times as far, feel 1/25 • Inverse-square law

  6. More inverse square • Spray paint can • Charge • Light from candle/match • Radiation • Sound • Examples on board • Gravity never 0, exerted over all space

  7. 7.5 Universal Gravitational Constant • F = Gm1m2/d^2 (to get the = sign) • G = 6.67E-11 Nm^2/kg^2 • Gravity is weak (we think strong because of all the things weakly pulling on us) • Circumference problem to illustrate G • Examples of G problems

  8. 7.6 Mass of Earth • We knew G 100 years after Newton’s theory of universal gravitation • Now we can calculate the mass of Earth! • F = 9.8 N, and pick a 1 kg mass on Earth

  9. 7.7 Ocean Tides • Tides affected by moon • Actually, differences between gravitational pull (weak with distance, remember?) and pushes on different parts • Low tide vs. high tide (2 per day) • Moon same position 24 hr 50 min (tides never same time) • Sun also affects tides, but 0.02 vs. 7 times as much. Why?

  10. More tides • Spring tides- sun, Earth, moon line up, higher than average tides and lower low tides (not Spring season)- new or full moon • ½ between new moon and full, tides partly cancel, lower highs, higher lows, neap tides • Tides unequal due to Earth’s tilt • No tides in lakes or us- no sig difference in moon pull (no piles of water) • Also, Earth has inner molten layer- tides in crust and atmospheric tides- cosmic radiation

  11. 7.8 Gravitation is Universal • Earth round because everything attracts everything else (come together, pull in corners) • Big Bang- 8-15 billion years ago, explosion of primordial fireball, infinite expansion or may come to halt with combined gravitation (Big Crunch). Might get new Big Bang • Get gravity from unseen- dark matter • Universe is young, we’re younger

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