Chapter 9 - Gravity

1. Chapter 9 - Gravity Tides Black Holes

2. Ocean Tides • Tides are caused by the difference in gravity between the moon & earth on opposite sides of the earth (different distances) • The force is stronger wherever the moon is closer to the earth and weaker wherever the moon is farther, and water can move easily! • The high tide bulge follows the moon around the earth

3. Tides • The sun also contributes to tides, but much less than the moon • The sun is less because the difference in distance from one side of earth to the other is not much when compared to the earth’s distance from the sun • The position of the sun, earth, and moon determines how high and how low tides are, along with the season of the year!

4. Tides • The highest high tides and lowest low tides happen when all 3 are in a line, or Spring Tides • Tide changes are smallest when they are not aligned, or Neap tides Neap Neap

5. Tides • High tides are even higher in winter because the earth’s center is closer to the sun in winter • The earth’s center is liquid, which results in “tides” in the earth’s crust • The atmosphere also has tides too! This is linked to “full moon” behaviors • The moon is slowing earth’s rotation

6. Earth’s Gravity—Below the Surface • What happens to gravity if you go down, inside the earth? • Gravity gets less and less the deeper you go because there is less mass. • Gravity at the center of the Earth is ZERO! • What would happen if there was a tunnel to the other side of the earth? • You would continuously oscillate back and forth from North to South Pole like game of ping pong!

7. Gravitational “Fields” • Force fields—gravity has a force field around any mass. It pulls other masses toward it and gets weaker with distance • Any force that acts at a distance, without contact, has a force field. Examples are magnetic and electric

8. Einstein and Black holes • Albert Einstein spent much of his life trying to explain what gravity is, and relate it to the other forces somehow • Visualized a gravitational field as a geometrical warping of four-dimensional space and time • “Don’t try. I can’t do it either.” -Einstein • His ideas explained Black Holes before they were found to exist

9. Black holes happen when a sun burns out and shrinks as it cools… Inverse-Square Law – if the star shrinks to 1/10th it’s original radius, the force of gravity would increase by 100 and so on. Stars mass does not change. Escape velocity increases and eventually exceeds the speed of light – nothing can escape! Black Holes

10. Universal Gravitation • Based on observing distant stars and galaxies, scientists confirm Universal Gravitation and use it to explain the Big Bang theory • Universe continues to expand. • Some scientists believe that eventually everything will stop moving apart and be pulled back together

11. Newton’s Influence • Newton’s laws changed science and civilization on earth • It can be tied to the industrial revolution • It can even be tied to the Declaration of Independence and Constitution through philosopher John Locke

12. Review Questions • Distinguish between spring tides and neap tides. • If you could somehow tunnel inside a star, would your weight increase or decrease? If, instead, you somehow stood on the surface of a shrinking star, would your weight increase or decrease? Why are your answers different?