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Homework Help: Circular Motion of a marble

  1. Oct 10, 2013 #1
    1. The problem statement, all variables and given/known data

    You swing a marble with mass m attached to the end of a string in a horizontal circle as shown in the figure below. The angle that the string makes with the vertical is
    θ = 37°.

    (a) Find the speed of the marble when the string is 26.0 cm long.

    2. Relevant equations


    Ar = v^2/r

    3. The attempt at a solution

    I solved for the radius by using trig, but now I don't know what to do. Without mass or revloutions, I don't know how to solve form the speed.
    Last edited: Oct 10, 2013
  2. jcsd
  3. Oct 10, 2013 #2


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    Draw the free body diagram: What forces act on the marble? What is the resultant when the marble moves along a horizontal circle? You do not need the value of the mass: it will cancel.

  4. Oct 10, 2013 #3
    You have gravity and force of tension
  5. Oct 10, 2013 #4


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    Yes, and what is their resultant?


    Attached Files:

  6. Oct 10, 2013 #5
    I actually have no idea. I'm blanking out.
  7. Oct 10, 2013 #6
    if its moving in circle, then some force should be providing centripetal force. Here that force is horizontal component of tension. And since the ball isn't moving up or down, force of gravity should be balanced by some other force, namely vertical component of tension.

    just as ehild said, making free body diagram would make it more clear.
  8. Oct 10, 2013 #7


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    The resultant of the tension and gravity is equal to the (horizontal) centripetal force. How is it related to mg and the angle of the string with the vertical ?

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