1. Chapter 11 – Gravity Lecture 1 April 6, 2010
Kepler’s Laws
Newton’s Universal Law of gravitation
Gravitational and inertia mass
Gravitational potential energy

2. Kepler’s (empirical) Laws
Kepler’s First Law:
All planets move in elliptical orbits
with the sun at one focus
Kepler’s Second Law:
The radius vector drawn from the Sun to a planet
sweeps out equal areas in equal times,
dA/dt = const (the law of equal areas)
Kepler’s Third Law:
The square of the orbital period of any planet is proportional to the cube of the semimajor axis of the elliptical orbit.
Today we can understand the physical reasons for these laws …
Let’s remind us first of the geometry of the ellipse and then discuss the three laws.

3. Ellipses
e = c/a

4. Planetary Orbits

5. Kepler found that the orbit of Mars was
an ellipse, not a circle.

8. Kepler’s Third Law
Kepler had access to very good data from the astronomer Tycho Brahe in Prague. See table for today’s data.
After many years of work Kepler found an intriguing correlation between the orbital periods and the length of the semimajor axis of orbits.

9. Of the satellites shown revolving around Earth, the one with the greatest speed is1 2 3 4 5

10. Of the satellites shown revolving around Earth, the one with the greatest speed is1 2 3 4 5
The constant-area law.

11. The orbits of two planets orbiting a star are shown
The orbits of two planets orbiting a star are shown. The semimajor axis of planet A is twice that of planet B. If the period of planet B is TB, the period of planet A is

12. The orbits of two planets orbiting a star are shown
The orbits of two planets orbiting a star are shown. The semimajor axis of planet A is twice that of planet B. If the period of planet B is TB, the period of planet A is

13. Newton’s Universal Law of Gravitym M
F12
F21 r

14. Newton’s Law of Gravity
Newton’s law of gravity will provide a physical theory of Kepler’s laws. m M
F12
F21 r
Magnitude of force

15. The Cavendish experiment

16. Measuring G G was first measured by Henry Cavendish in 1798
The apparatus shown here allowed the attractive force between two spheres to cause the rod to rotate
The mirror amplifies the motion
It was repeated for various masses
9/17/2018
Physics 201, UW-Madison

17. Gravitational and inertial mass
Gravitation is a force that acts on the gravitational mass
(the masses are the source)
Newton’s Law of motion acts on the inertial mass
In principle, they are not necessarily related, that the gravitational mass mg is not the same as mi
(but they are, up to the current experimental accuracy)

18. Kepler’s Third Law derived from Newton’s Law
Easily derived for a circular orbit
Centripetal force = gravitational force
Kepler’s Third Law

19. Kepler’s Third Law derived from Newton’s Law
Extension for elliptical orbits,
(Without proof R  a)
Kepler’s Third Law
Where a is the semimajor axis

21. A woman whose weight on Earth is 500 N is lifted to a height of two Earth radii above the surface of Earth. Her weight
decreases to one-half of the original amount.
decreases to one-quarter of the original amount.
does not change.
decreases to one-third of the original amount.
decreases to one-ninth of the original amount.

22. A woman whose weight on Earth is 500 N is lifted to a height of two Earth radii above the surface of Earth. Her weight
decreases to one-half of the original amount.
decreases to one-quarter of the original amount.
does not change.
decreases to one-third of the original amount.
decreases to one-ninth of the original amount.

23. From work to gravitational potential energy.
In the example before,
it does not matter on what path the person is elevated to 2 Earth radii above.
Only the final height (or distance) matters for the total amount of work performed.

24. Potential Energy m M r 2 1
Work done to bring mass m from initial to final position.
Zero point is arbitrary. Choose zero at infinity. 1

25. Demo