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Circular motion/gravitational force implied

  1. Oct 24, 2008 #1


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    1. The problem statement, all variables and given/known data
    I've thought about a problem that I invented but couldn't solve it. So I'd like a very little help, something that can push me in the good direction but not the full answer.
    Suppose we have a system that is composed of a planet and a body. The body is at an height h from the center of the planet (of course h is greater than r). Initially the body is at rest. What is the impulse we have to apply on this body in order to make it move in such a way so that it describes a circular path around the planet? Give the answer in terms of m (the mass of the body), R (distance between the center of the planet and the body), M the mass of the planet and so on. With the impulse, I can then calculate the velocity it must have to accomplish this task.
    Thank you!

    2. Relevant equations

    3. The attempt at a solution
    I've tried a few things, but I'm at a loss.
  2. jcsd
  3. Oct 24, 2008 #2
    If you are looking for velocity you can easily get it if you have M (big mass) and the h

    to do that make Fc=Fg and isolate v :)
  4. Oct 24, 2008 #3


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    Thanks a bunch Epsillon!! I had been stuck for a few days and I just can't believe I missed such a simple answer. I thought it would have been much more complicated. I didn't realize that I could get rid of the centripetal acceleration by considering that it's equal to the velocity squared over r.
    So finally I found that the velocity is worth [tex]\sqrt{\frac{GM}{r}}[/tex].
  5. Oct 24, 2008 #4
    You got it!
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