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# Unit 16 - The Universal Law of Gravity

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• ### Unit 16 - The Universal Law of Gravity

1. 1. The Astronomy Picture of the Day
2. 2. The Universal Law of Gravity
3. 3. Outline • Orbital Motion & Gravity • Newton’s Universal Law of Gravitation • Surface Gravity & Weight
4. 4. (A) A cannon on a mountain peak fires a projectile. If the projectile is fired faster, it travels farther before hitting the ground. (B) At a sufficiently high speed, the projectile travels so far that the Earth’s surface curves out from under it, & the projectile is in orbit.
5. 5. Gravity produces a force of attraction (green arrows) between bodies. The strength of the force depends on the product of their masses, m & M, and the square of their separation, d. G is the universal gravitational constant.
6. 6. Astronaut John Young making a jumping salute at the Apollo 16 lunar landing site near Descartes Crater. Despite a total mass of over 170 kg (~370 pounds) in his space suit, he easily jumps up because of the Moon’s weak gravity.
7. 7. Center of Mass The terms "center of mass" and "center of gravity" are used synonymously in a uniform gravity field to represent the unique point in an object or system which can be used to describe the system's response to external forces and torques. The concept of the center of mass is that of an average of the masses factored by their distances from a reference point. In one plane, that is like the balancing of a seesaw about a pivot point with respect to the torques produced.
8. 8. Key Terms • g: the acceleration due to gravity at Earth’s surface—about 9.8 m/s2. • G: Newton’s gravitational constant, which allows us to determine the force between objects if we know their masses & separation. • newton (N): the standard unit of force in the metric system. • Surface gravity: the acceleration an object will experience near the surface of a planet (or other body) because of the planet’s gravitational pull.
9. 9. Law of Gravity • A description of the gravitational force exerted by one body on another. It is proportional to the product of their masses (M, m) & the inverse square of their separation (d): FG = M ×m G d2 G = 6.67×10-11 m3-kg-1-s-2
10. 10. Newton’s cannon
11. 11. Force of Gravity