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Homework Help: Some Circular Motions Questions (No math)

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data
    1) If the earth could be made to spin so fast that ac=g, how would this affect the apparent force of gravity on you?

    2)Why doesent water in a bucket spill out when it is in circular motion?

    3) What happens to a sattelite when Fg is bigger than Fc and Fg is smaller than Fc

    3. The attempt at a solution

    1) We would be weightless. But I am not sure about the reason WHY. is it because the earth is falling faster than we are ?

    2) The bucket is falling at the same rate as water and therefore it catches it?

    3) Fc=fg it orbits and if its fg is bigger will it go out of orbit and smaller crash into earth? Why is this? This one I have no idea.
  2. jcsd
  3. Oct 20, 2008 #2


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    Hi Epsillon! :smile:

    (i take it c means centripetal and g means gravity)

    2) If "no math" in the title is part of the instructions, then your answer is fine …

    except you might want to throw in the word "acceleration" a few times, just to prove you understand what it is! :wink:

    1) Consider the acceleration (and it may help you to think of the Earth as a hollow sphere, and we're standing on the inside of it :wink:)

    3) Hint: will it only stay in orbit if Fc exactly equals Fg? :smile:
  4. Oct 20, 2008 #3
    Thanks For the Respons!

    No the questions never said that no math is required but our teacher is somewhat picky and hates students that just do all math and dont understand concepts.

    So I still do not get 1. Like what if we are on the pole????

    I assume if you are on the pole it makes no difference but being exactly on the equator I kinda disagree with my previous statement now because I think that we actually arent falling at all? (Is this true?)

    I still cant explain 1 properly but just imagining it i think we will be weightless.

    Can you prove it using math?

    For 2 I think I get it completly now and thanks for pointing out that I didint use acceleration at all :)

    3 I thought about it some more and now I am more confused.

    If Fg is bigger wouldnt it just crash into the earth??

    And smaller wouldnt it just be attracted to another planet or something and go off??

    Oh and about you hint/question. Obviously it is impossible to make Fg exactly Fg so how does it work???
  5. Oct 20, 2008 #4
    Oh so Fc exactly doesent equal Fg does it go in a eliptical shape?
  6. Oct 21, 2008 #5


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    Yes!! :biggrin:

    If Fc is too small, the ellipse will intersect the Earth's surface …

    a crash is just an ellipse that's too thin! :wink:

    And if Fc is too large, it may … ? :smile:
    Fc must mean the local acceleration, so it obviously doesn't apply at the pole.

    If the surface of the Earth wasn't there (at our particular place), we would be falling.

    But that's not really a maths explanation …

    for that, use good ol' Newton's second law … F = ma. :smile:
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