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Homework Help: Tension in a pendulum (probably just me being stupid)

  1. Mar 25, 2010 #1
    1. The problem statement, all variables and given/known data
    A pendulum consisting of a small heavy ball of mass m at the end of a string of length l is released from a horizontal position. When the ball is at point P, the string forms an angle 30 degrees with the horizontal.
    a. Free body diagram (all set on that)
    b. Determine the speed of the ball at P.
    c. Determine the tension in the string when the ball is at P.
    d. Determine the tangential acceleration of the ball at P.

    2. Relevant equations
    PEi + KEi = PEf + KEf
    a = v^2 / r
    Fc = mv^2 / r

    3. The attempt at a solution
    a. was just drawing a free body diagram, easy. For b., PE = KE, mglsin(30) = .5mv^2, v^2 = gl, v = sqrt(gl).

    For c., is it just that the tension is equal to the centripetal force? In that case, Ft = Fc = mv^2/r = mv^2/l = mg? It just seems strange that it's the same as the force due to gravity at this point.
  2. jcsd
  3. Mar 27, 2010 #2
    Still need some help
  4. Mar 27, 2010 #3
    Yeah, I am 100 per cent positive that the centripetal force is equal to the tension.

    Concerning your remark, it is just a coincidence that Fc appears to be equal to mg, but note that this only happens when the angle is 30 degrees.
  5. Mar 28, 2010 #4
    Centripetal force is not just tension, it also includes the relevant component of gravity at P.
  6. Mar 28, 2010 #5
    Ops yeah you're totally right, my bad. Those are the two forces acting in that very moment; both gravity component at that angle and F_c.
  7. Mar 28, 2010 #6
    Isn't gravity just acting straight down, and the centripetal force is the center-pointing force, the force pulling it in and causing it to "rotate" rather than just fall?
  8. Mar 28, 2010 #7
    No, think of it this way: when the pendulum bob reaches the lowest point (90º from the horizontal) it is clear that gravity will play an important role to the tension at that point, so it is at point P.
  9. Mar 28, 2010 #8
    But isn't that because at that point ALL of the force due to gravity is canceled by the tension force (the centripetal force) whereas at point P it's only a component?
  10. Mar 28, 2010 #9
    No, If you draw the FBD you will see how the part of the gravitational force opposite to the tension must cancel the force of the tension in the string when the pendulum is at rest. But if we have an additional component, such as the centripetal force, the tension must be greater to provide sufficient stability, not letting the bob to break apart. That lead us to think that:

    [tex] T = F_c + mg\cdot cos\theta [/tex]
  11. Mar 30, 2010 #10
    I just don't get what you mean by part of the gravitational force. The gravitational force is just straight down, no? The tension is the one with components isn't it? At rest, Fg = Ft obviously. Fc isn't an additional component is it? I thought it was CAUSED by an already present force...ie tension?
  12. Mar 30, 2010 #11
    Do you have a knowledge of vectors?
  13. Mar 30, 2010 #12
    Yes, but I tend to be kind of shaky with them except in basic addition and subtraction.
  14. Mar 30, 2010 #13
    Then you would have learnt vector resolution too - breaking the vector into mutually perpendicular independent components? Try this with gravity with one component along the direction of motion of pendulum and one perpendicular to it. What do you get?
  15. Mar 30, 2010 #14
    A translated version of the tension FBD, so I set Ft equal to mgsin30? I understand that the math behind this works I guess, I just don't quite "see" it, I would have never thought to do that...anyways...

    Fc = Ft - Fgsin30 = mv^2/r
    mgl/l + .5mg = Ft
    Ft = 3mg/2?
  16. Mar 30, 2010 #15
    Well, not equal, since mv^2/r isn't 0, but you get the idea...I'm a little tired.
  17. Mar 30, 2010 #16
    Relax. These things become clearer with time. But I think you have the answer. Just keep in mind that the second law is a vector law and valid along directions.
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