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Astronomy 102, Fall 2003

Homework Assignment #2

This assignment is due at the beginning of class on Friday, September 12. Late homework will not be accepted.

You must do the first three problems. Each of those problems will be worth 10 points. If you make a sincere, honest effort to answer each question, you will receive at least 5 points of credit. Do not abuse this, or I will stop doing it later in the term!

Staple your homework! If you require more than one page to complete the homework, fasten the multiple pages together with a staple; folding the corner won't cut it. If your homework has multiple pages but you fail to staple, you will be docked 3 points.

The last three problems are given to you as additional review problems. You do not need to turn them in, and they will not be graded when you do. However, solutions to them will be posted along with the solutions to the first three problems. You may want to do them if you think you need extra review in the class.

Please write out the problem statement at the top of your solution. (This is for two reasons; it is so I can know which problem you answered, and that you answered the right problem from the book. It also will make your graded homework more useful as a study aid later.)

  1. The moon and all of the planets appear to go through phases, meaning that most of the time we see only a part of their surface illuminated. The Sun never goes through such phases. Explain why the Sun is different from the Moon and the planets

  2. Chapter 3, Question 11 (page 83).

  3. Chapter 3, Question 17 (page 84). Give your answer in both Newtons and Pounds

    (The problems below will not be graded, and need not be turned in.)

  4. One of the most famous comets in the solar system is Comet Halley. comet. This comet returns to the inner solar system and is visible every 76 years; it's last apparition was in 1986. The comet, like other periodic comets, is in a highly eccentric elliptical orbit around the Sun.

    • (a) What is the semi-major axis of the orbit of Comet Halley?
    • (b) Comet Halley is typically only visible for several months out of the 76 years of its orbit. Use Kepler's laws to explain why.
    • (c) When should we expect to see Comet Halley again?

  5. You are standing on the surface of the Earth. Calculate the gravitational force on you (assume you mass 75kg if you don't know your mass) from (a) the Earth, (b) the Sun, (c) the Moon, (d) Mars, and (e) Jupiter. Assume that Mars and Jupiter are each about as close as they get to the Earth (0.5 AU and 4.2 AU respectively). Here are some more numbers you will need:

    • Radius of the Earth: 6400km
    • Mass of the Earth: 6.0x1024 kg
    • Distance from the Sun to the Earth: 1 AU
    • Mass of the Sun: 2.0x1030 kg
    • Distance from the Earth to the Moon: 380,000 km
    • Mass of the Moon: 7.4x1022 kg
    • Mass of Mars: 6.4x1023 kg
    • Mass of Jupiter: 1.9x1027 kg

    (Remember to convert units to a consistent system to get answers that make sense!)

    For comparison, calculate the gravitational force on you from a car (2000kg) that is 1m away from you. Think about this next time somebody tells you that the motions of the planets, or their position at your birth, may affect your character or your daily life...


Last modified: 2003-September-08 , by Robert A. Knop Jr.

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