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Seesaw and torque

  1. Mar 30, 2008 #1
    When I do some physics derivation, I find that on a seesaw, if the object is farther away from the fulcrum, the angular acceleration decreases. Is this true? If not, where did I go wrong?
    [tex]\tau = I \cdot \alpha[/tex]

    [tex]\tau=F \cdot r[/tex]

    Then, [tex]\alpha = \frac{F \cdot r}{I} = \frac{F_g \cdot r}{mr^2}=\frac{g \cdot m \cdot r}{mr^2}=\frac{g}{r}[/tex], so if the distance "r" increases, angular acceleration decreases...did I do something wrong?
    Last edited: Mar 30, 2008
  2. jcsd
  3. Mar 30, 2008 #2


    Staff: Mentor

    That is correct.
  4. Mar 30, 2008 #3
    Are you sure? Some people have told me that it's wrong, because I have to account for the mass of the seesaw.
  5. Mar 30, 2008 #4


    Staff: Mentor

    It's not that it is wrong, it simply makes some assumptions. The formula you derived is for a point mass on a massless seesaw. If your seesaw is not light enough to approximate as massless or if your load is too large to be considered a point then your formula doesn't apply.
  6. Mar 30, 2008 #5
    What if I cannot assume the masses are negligible? How would I derive that? And in that case, would angular acceleration increase?
  7. Mar 31, 2008 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Just include the rotational inertia of the seesaw as part of the total rotational inertia:
    [tex]I_{total} = I_{seesaw} + mr^2 = 1/12 M L^2 + mr^2[/tex]
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