Transcript Document

Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(Please use PHYS214 in e-mail subject)
Movie + Lecture 3. Exercise on Kepler’s laws
(Page 28-30)
January 11
Lecture 3
Textbook: Pages 28 and 30
• Movie on Kepler’s laws
• Kepler`s third law calculations
Movie on Kepler’s laws
“We are star stuff!”
Cosmos series
(Emmy award winning
series)
By Carl Sagan
Part III: The harmony of
the world.
Episode 8 (35 min)
Kepler’s Third Law
Kepler’s third law: More distant planets orbit the Sun at slower
average speeds, obeying the precise mathematical relationship.
p2=a3
where p is the planet’s orbital period in years, and a is the average distance
(semimajor axis) from the Sun in astronomical units (AU).
1 AU = Earth’s average distance from the Sun about 149.6 million km
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Kepler’s law applies to any orbiting object as long as the following two
conditions are met:
The object orbits the Sun or another star of exactly the same mass.
We use units of years for the orbital period and AU for the orbital
distances.
Kepler’s Third Law
Kepler’s third law:
p2=a3
where p is the planet’s orbital period in years, and a is the average distance (semimajor
axis) from the Sun in astronomical units (AU).
Kepler’s Third Law
Exercise 1. (similar to exam question):
The largest asteroid, Ceres, orbits the Sun at an average distance (semimajor axis) of a
= 2.77 AU. What is Ceres orbital period p?
p 2  a 3  p  a 3  2.77 3  4.6
Answer: Ceres has an orbital period of 4.6 years
Exercise
2. (similar to exam question):
• A planet is discovered orbiting every three month around a star of the same mass as
the Sun. What is the planet average distance a?
p 2  a 3  a  3 p 2  3 0.252  0.4
Answer: The planet orbits its star at an average distance of 0.4 AU.20
Next Lecture
Modern science and essential concepts of physics & astronomy
Textbook: Pages 33-43, 81-88
MOVIE. Hubble - 15 years of discovery (15 minutes)
Chapter 5. Monsters in space, black holes, worm holes, gamma rays.
Chapter 6. Gravitational lensing