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Homework Help: Gravitational force exerted by Jupiter on a baby

  1. Feb 18, 2014 #1
    Hi everyone, I'm a college student enrolled in an astronomy course, and simply put, science and math in general are both quite possibly the two things I'm worst at! Anyways, I have a test coming up and our professor gave us a practice test, and after a lot of studying, I was able to figure out most problems. This one, however, I have spent the past two ours trying to solve with no luck:

    Suppose you wanted to calculate the gravitational force exerted by Jupiter on a 4.0-kg baby at the
    moment of her birth. What is the minimum distance possible distance between the baby and Jupiter?
    Under what circumstances would that occur? What would be the magnitude of the gravitational force
    Jupiter exerts on the baby under those conditions?

    It's from the chapter on Kepler's three laws, and I know that at some point this equation should be used:
    Fg=G x mm (mass of two objects) divided by distance squared

    As said before, this is a practice problem, so I'm not trying to get anyone to solve my homework for me, I was just hoping someone here could walk me through this! Thanks!
  2. jcsd
  3. Feb 18, 2014 #2


    Staff: Mentor

    Welcome to PF!

    First you should use the homework template for your question and provide some work to show what you're thinking.

    To start things off, I assume the baby is being born on the Earth and is being affected by Jupiter's gravity.

    What is the closest distance that the Earth can be to Jupiter? You can find this online.

    Given that then assume the Jupiter is directly overhead so that the baby is affected by Earth's gravity downward and Jupiter's gravity upward.

    Does that help?
  4. Feb 18, 2014 #3


    User Avatar
    Science Advisor
    Gold Member

    The question doesn't ask for the net force on the baby, just for the baby-Jupiter gravitational force.

    tchls1, you've got the right equation there. Just plug the numbers in and you'll get the magnitude.

    The other two questions don't require any math. Best to draw the orbits of Earth and Jupiter around the sun to help you visualise the situation and see the conditions under which the distance is the closest.
    One other thing to consider would be what does treating the Earth as a point or as a sphere change in the picture, and if it's a change that matters.
  5. Feb 18, 2014 #4


    Staff: Mentor

    Thanks for the clarification, I was simply trying to setup the scenario and let the op decide what to do next.
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