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What happens to an object in a free-falling ship?

  1. Dec 5, 2015 #1
    The answer to this question should be obvious, but I want to make sure. I'm writing a quiz containing difficult general-science questions. One of the questions is:

    "You’re in an unpowered spaceship that’s going around the Sun in a very off-kilter (i.e., highly eccentric) orbit. Just as your ship is tearing around the Sun at its closest approach, you release a ball inside the ship. What does the ball do?"

    I believe that it floats. I assume tidal effects can be considered nil or negligible.

    I like the question because everybody knows the case of the free-falling elevator. In this case, a lay reader might be tempted to believe there's something different about bending around the Sun, as opposed to falling directly toward it. So it checks whether they know that orbiting is free falling.

    Thank you!
  2. jcsd
  3. Dec 5, 2015 #2


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    But if the ship is large enough, tidal effects may not be zero everywhere in the ship.
    It can't hurt to specify that the ball is released at/near the centre-of-mass of the ship.

    Also, eliminate the term off-kilter; it may lead to confusion.
  4. Dec 5, 2015 #3
    You should mention that the ship is non-rotating.
  5. Dec 5, 2015 #4
    Ah that's an excellent point and exactly why I came here.

    I assume if I specify it's at the center of mass, that also makes rotation a non-issue.
  6. Dec 5, 2015 #5
    That depends on the system of reference.
  7. Dec 5, 2015 #6


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    If the ball is at the CoM, it will stay there, whether or not the ship is rotating (since, after-all, it will be rotating about its CoM).
  8. Dec 6, 2015 #7
    That would require that the ball is released without initial speed.
  9. Dec 6, 2015 #8


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    The entire experiment is predicated on the ball being released without initial speed.
    I'll bet that the answer the OP is expecting is 'the ball will remain stationary', not 'the ball will continue on its course.'

    If you assume some initial motion, yes, you've got some serious problems. Now you have to describe what it means for the ship to be non-rotating. Relative to the Sun? Relative to the fixed stars?

    Yes, we know the answer, but the problem would have to be explicit about it to avoid confusion, and that adds complexity to what is supposed to be a simple question.

    So yup, you're going to have specify that the ball is released without any initial motion.
  10. Dec 6, 2015 #9
    Even after second reading I did not find such a limitation.

  11. Dec 6, 2015 #10
    It really is supposed to be a simple question for the general public. I think complications such as an initial speed, rotation, etc. will be eliminated by the other choices of answers -- things like "It moves toward the Sun due to the strong gravity" and "It moves away from the Sun due to centrifugal effects." Anyhow, thanks for your help as always.
  12. Dec 6, 2015 #11


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    I know. It's implicit. You introduced the element of movement which means you then have to address the element of rotation. Which makes it more complicated. Since the OP expects it to be simple then we're gonna have to make sure it's simple. The OP's queston and answer are predicated on being simple.


    Which is it? Relative to the Sun or relative to the fixed stars? Either one is a reasonable understanding from a general public person. This is demonstrable right here on PF considering the number of people that argue till they're blue that the Moon is not rotating as it orbits Earth.

    Yup. It just takes a little careful wording up front to ensure it is simple.
  13. Dec 7, 2015 #12
    Relative to an inertial system. Rotation doesn't need an external reference.
  14. Dec 7, 2015 #13


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    As I said, while true, it will complicate the question for the reader, leading to confused, incomplete or incorrect answers, confounding the OP's goal. It's supposed to be simple - weightlessness in solar orbit being similar to weightlessness in Earth orbit. That's really all they're supposed to deduce.
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