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Homework Help: Circular movement

  1. Jul 11, 2008 #1
    1. The problem statement, all variables and given/known data

    Everything is in the pictures.
    I did part A, I have a problem with part B.
    10x in advance.

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jul 11, 2008 #2

    Doc Al

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    Staff: Mentor

    You need to find the tension as a function of angle. (Forget the tension at the top.)

    You have the correct equation. Hint: Express v^2 as a function of angle.
     
  4. Jul 11, 2008 #3
    I know this equation (in the pic) but I'm doubtful if this what you meant...

    Should I use trigo? can I get another tip :smile:?

    10x.
     

    Attached Files:

    • 1.jpg
      1.jpg
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  5. Jul 11, 2008 #4

    Doc Al

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    The equation I had in mind was: mv^2/r = T + mg cos(theta).

    Another hint: What's conserved as the ball continues on its path?
     
  6. Jul 11, 2008 #5
    Ok, I already wrote that equation at the beginning in the solution part I uploaded and got stuck there.

    I got stuck because I dont know the velocity at this point, I mean, the velocity is not constant, right? there is a mgsin(theta) that keep changing it...I think.

    The velocity at the top is not the velocity at the bottom, right?

    10x.
     
  7. Jul 11, 2008 #6

    Doc Al

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    The velocity is definitely not constant. Reread my hint in post #4. :wink:

    Given the velocity at the top, you should be able to find the velocity at any point as a function of angle.
     
  8. Jul 11, 2008 #7
    :confused: I'm totally stuck, should I use trigo, or there is some equation for the velocity that I dont know about...

    Oh, maybe u mean to use energy calculation?
     
  9. Jul 11, 2008 #8

    Doc Al

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    Getting warmer! :smile:
     
  10. Jul 11, 2008 #9
    The potential energy turns into kinetic energy, no?
     
  11. Jul 11, 2008 #10

    Doc Al

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    You got it. Keep going.
     
  12. Jul 11, 2008 #11
    Ok, I haven't really learned Conservation of energy so I'm not really familiar with it...

    I wrote this equation about the energy, is it correct?

    And another thing, What about T? it's not a Conservative force, right? Do I just ignore it in the the energy equation?

    10x.
     

    Attached Files:

  13. Jul 11, 2008 #12

    Doc Al

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    Looks good to me.
    Excellent question! Ask yourself: Does the tension force do any work on the ball?
     
  14. Jul 11, 2008 #13
    Oh, right, it's vertical to his movement.

    10x a lot.
     
  15. Jul 12, 2008 #14

    Doc Al

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    The ball's movement is always perpendicular to the tension, thus the tension does no work on the ball.
     
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