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Bar with masses on ends

  1. Oct 6, 2011 #1
    1. The problem statement, all variables and given/known data
    A machine part consists of a thin 35.1-cm-long bar with small 1.81-kg masses fastened by screws to its ends. The screws can support a maximum force of 70.3 N without pulling out. This bar rotates about an axis perpendicular to it at its center.

    2. Relevant equations
    Newton's 2nd law, a_rad=v^2/r

    3. The attempt at a solution
    (What am I doing wrong?)
    The centripetal acceleration is v2/r=v2/0.351 m/s^2, so the net force on each mass is 1.81v^2/0.351 N ≤ 70.3 N

    Solving we get v ≤ ~3.69226 m/s, so the largest v is that.

    However, I'm being told this is wrong. However, the same problem with very similar numbers but different numbers had an answer of ~3.61 which is pretty close.
  2. jcsd
  3. Oct 6, 2011 #2
    Are you sure http://en.wikipedia.org/wiki/Centripetal_force" [Broken] is the right force?

    Gravity is attempting to pull the weight down, which would turn the bar.

    If the bar turned, the weight would get closer to the axis of rotation.

    Is there a force that might be against the weight getting any closer to the axis of rotation?

    Last edited by a moderator: May 5, 2017
  4. Oct 6, 2011 #3


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    What you have done seems to be correct, that is, if you were asked the maximum speed of the masses.

    Are you sure it doesn't ask for the maximum rotational speed (angular velocity) ?
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