Planets in Their Orbits

1. Planets in Their Orbits Kepler and Newton

2. Kepler • Kepler, using his and Brahe’s data, established three laws of the motions of the planets. • We have already discussed the first.

3. I. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

4. Kepler • Kepler’s second law was about how the planets traveled in their orbit around the Sun. • His measurements indicated that the planets did not travel at a constant speed around the Sun. • Planets slowed as they moved away from the Sun and speed up as they get close to the Sun.

5. Kepler • What he discovered was that lines joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.

6. http://kingfish.coastal.edu/physics/physlets/Orbits/kepler_2nd_law.htmlhttp://kingfish.coastal.edu/physics/physlets/Orbits/kepler_2nd_law.html Show orbit with sweeps.

7. Kepler • This is called Kepler’s second Law. • Kepler was able to mathematically describe this movement but not what caused it. • This had to wait for Isaac Newton.

8. Newton • Prior to Newton people thought objects at rest would remain at rest • And • Objects in motion would come to rest.

9. Newton • Newton changed that with his first law of motion. • We often refer to this as inertia.

10. Newton • Newton was also able to mathematically explain gravity as a force that depended on the size of an object and the distances between the objects.

11. Newton • Newton’s second law of motion also figures in motion of the planets. • F = m a • Where F = force, m = objects mass and a = acceleration

12. Newton • When Newton put this information together his calculations for the motions of the planets matched Kepler’s. • Simply stated planets orbit because of the balance between gravity trying to pull them into the Sun and inertia trying to throw them out into space.

13. As the planet falls toward the Sun its velocity increases so the force trying to push the planet out into space increases as the gravitational force increases. • As the planet whips around the Sun it is at its maximum velocity and Kinetic energy.

14. As the planet moves away from the Sun its speed is decreased by the pull of the Sun’s gravity. • At aphelion the planets speed is slowest and it starts its fall back to the Sun. • This is when it is at maximum Potential energy.

15. Since there is no other force in space to interfere with this balance of potential and kinetic energy it will keep moving for ever.