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Centripetal Motion

  1. Jan 17, 2009 #1
    I have a problem that I've tried to figure out, but I can't figure it out.

    Here it is:

    You whirl a ball of mass 0.40 kg on a string of length 0.90 m. At the point shown the angle of the string from the vertical is 30 degrees and the ball has a speed of 3.5 m/s.

    a) Make a free body diagram of the ball.
    b) Find the tension in the string.
    c) What is the acceleration of the ball, tangential and radial components?
    c) Now consider the ball at the top of the circle. What is the smallest speed of the ball so that the ball continues in a circle?

    Any help with this problem would be greatly appreciated!
     
    Last edited: Jan 17, 2009
  2. jcsd
  3. Jan 17, 2009 #2
    I have a homework problem that I've tried to figure out, but I can't figure it out.

    Here it is:

    You whirl a ball of mass 0.40 kg on a string of length 0.90 m. At the point shown the angle of the string from the vertical is 30 degrees and the ball has a speed of 3.5 m/s.

    a) Make a free body diagram of the ball.
    b) Find the tension in the string.
    c) What is the acceleration of the ball, tangential and radial components?
    c) Now consider the ball at the top of the circle. What is the smallest speed of the ball so that the ball continues in a circle?

    Any help with this problem would be greatly appreciated!
     
  4. Jan 17, 2009 #3

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi rickylin89! :wink:

    Show us what you've tried, and where you're stuck, and then we'll know how to help. :smile:

    (and please don't double-post :frown:)
     
  5. Jan 17, 2009 #4
    Honestly, I really don't know how to start this problem. Any help would b great.
     
  6. Jan 17, 2009 #5
    Draw a free body diagram and write the equations of motion. If you are familiar with them, us the polar coordinate form for the equations of motion; it will make things easiest.
     
  7. Jan 17, 2009 #6
    I'm not sure if I have the right free body diagram. I figured there was a down force of gravity, an up force of net force, and then I don't know what other forces there are.
     
  8. Jan 17, 2009 #7
    Where does this "up force" come from? The only forces you can include have to come from somewhere!

    What direction is the force in the string?
     
  9. Jan 18, 2009 #8
    I think there is a tension that is 60 degrees to the left of the down force of mg. Is that righ?
     
  10. Jan 18, 2009 #9
    What matters is that the tension in the strings is ALONG the strong.
     
  11. Jan 18, 2009 #10
    I don't really understand what you mean.
     
  12. Jan 18, 2009 #11
    The tension is along the string, but I was referring to the free body diagram and the fact that the tension is mgsin(60). Is that right?
     
  13. Jan 18, 2009 #12
    No, that is really not correct. Just call the tension T for right now, but recognize that it acts along the string. The apply Newton's second law, with the proper acceleration of the mass and the correct force sums along the string and perpendicular to the string.
     
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