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# Gravity

Gravity. Ch 11. Kepler’s Laws. LAW 1: All planets move in elliptical orbits with the Sun at one focus. LAW 2: A line joining any planet to the Sun sweeps out equal areas in equal times.

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## Gravity

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### Presentation Transcript

1. Gravity Ch 11

2. Kepler’s Laws LAW 1: All planets move in elliptical orbits with the Sun at one focus.

3. LAW 2: A line joining any planet to the Sun sweeps out equal areas in equal times.

4. LAW 3: The square of the period of any planet is proportional to the cube of the semimajor axis of its orbit. C has the same value for all planets b/c force exerted by Sun on a planet varies inversely with the square of the distance from the Sun to the planet. Ellipse: Locus of points for which r1+r2=constant Semimajor axis: distance a Semiminor axis: distance b

5. Jupiter’s mean orbital radius is 5.2AU. What is the period of Jupiter’s orbit around the Sun?1 AU is the mean distance from Earth to Sun.

6. Newton’s Law of Gravity • Newton proved that an attractive force that varies inversely with the square of the distance between the Sun and a planet would result in an elliptical orbit, as observed by Kepler. • Bold assumption: This attractive force acts between any two objects. • Newton showed that laws of physics apply equally well to both terrestrial and nonterrestrial objects

7. Show that the gravitational force that attracts a 65kg man to a 50kg woman when they are .50m apart is 8.7X10-7N.

8. This demonstrates that the gravitational force exerted by an object of ordinary size on another such object is so small as to be unnoticeable. Calculate the force of attraction between two mosquitos that are .50m apart weighing 1X10-7N each. That’s the weight of about 9 mosquitos!!! A 50kg woman weighs about 490N – Half a billion times greater than the force between the man and woman.

9. What would be the free fall acceleration of an object at the altitude of the space shuttle’s orbit, about 400km about the Earth’s surface?

10. Now let’s redefine LAW 3

11. You are trying to view the International Space Shuttle, which travels in a roughly circular orbit around the Earth. If its altitude is 385km about the Earth’s surface, how long do you have to wait between sightings? Earth’s mass: 5.98X1024kgEarth’s radius: 6375km

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