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Homework Help: Centripetal Motion Problem Please Help

  1. Sep 19, 2006 #1
    ok so i've got a problem i cannot figure out and i know i'm skipping some simple step, so could someone please help me with the following question:

    Estimate the force a person must exert on a string attached to a 0.190 kg ball to make the ball revolve in a horizontal circle of radius 0.600 m. The ball makes 1.40 revolutions per second. Do not ignore the weight of the ball. In particular, find the magnitude of FT, and the angle it makes with the horizontal. [Hint: Set the horizontal component of FT equal to maR; also, since there is no vertical motion, what can you say about the vertical component of FT?]
    FT = ?N
    Theta= ?Degrees

    There's an attached picture to go along with the problem.

    Thanks for any help you can give me.

    Attached Files:

  2. jcsd
  3. Sep 19, 2006 #2


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    Staff Emeritus
    Science Advisor

    One can draw vector diagram.

    The weight, mg, acts downward (with gravity).

    The centripetal force acts outward, normal to gravity.

    The resulting force vector along the string must be opposite and equal in magnitude to the resultant force of weight and centripetal force.
  4. Sep 20, 2006 #3
    I'm still having trouble getting the right answer. I think i'm starting off on the wrong track or something. I have:

    Sum Fy= 0 = m(v^2/r)sin@-mg

    this gives me the wrong angle. Is there a better way to solve for Ft first?
  5. Sep 20, 2006 #4
    Actually yeah i just got it i was trying to break the Fc into components so i was messing up. Thanks for your help
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