AST 307 · Introductory Astronomy
Fall 2003

AST 307
Homework #2
Due Friday Sep. 12

  1. We talked about the apparent motion of Mars on the celestial sphere in class. It usually moves in the prograde direction, but moves appears to move retrograde when we are passing it. Draw a picture showing Venus and the Earth orbiting the Sun, and use it to figure out when Venus moves prograde and retrograde.

  2. Halley's comet has a perihelion distance (closest distance to the Sun) of 0.36 AU and an aphelion distance (farthest from the Sun) of 36 AU. Its speed at perihelion is 70 km/sec. (I made up some round numbers to make the answers come out reasonably simple. They aren't quite right, but they're close.) Use Kepler's laws to answer the following questions.

    a) What is the semimajor axis, a, of Halley's orbit?
    b) What is Halley's speed at aphelion?
    c) What is the period of Halley's orbit?
    d) What is the speed of an object in circular orbit 0.36 AU from the Sun?
    e) What is the speed of an object in circular orbit 36 AU from the Sun?
    f) Notice how these last two speeds compare to Halley's speed at those
    two distances. Is the difference sensible (qualitatively)?

  3. Assume you are in a spaceship floating in space far from any stars or planets, and you have your rocket engines off. Assume your mass is 50 kg, the mass of your spaceship is 500 kg, and the length of your spaceship is 10 m.

    a) If you push off from one end of the spaceship with a force of 10 Nt, what is your acceleration while you are pushing?
    b) If you push for one second, what is your speed after you stop pushing and start coasting away from the end of the spaceship? Assume you and the spaceship started out with zero velocity.
    c) At this time what is the speed of the spaceship?
    d) You coast down to the other end of the spaceship. How long does it take for you to get to the other end? Ignore your height.
    e) When you reach the other end of the spaceship, you push just hard and long enough to come to a stop relative to the spaceship. At this point what is the speed of the spaceship?
    f) How far did the spaceship move during all of this?

    If you didn't take the motion of the spaceship into account in determining how long it took you to coast the length of the ship, go back and redo that question taking it into account.

9 September 2003
Astronomy Program · The University of Texas at Austin · Austin, Texas 78712
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