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Homework Help: Stuck on a few Physics Problems

  1. Oct 7, 2004 #1
    1. Four horses together pull an old-fashioned drill of the type used to make big guns. It has four levers mounted horizontally on a revolving central axes. Assuming each horse pulls at the same angle of 45 degress to the radial lever of the drill, and pulls with a given force, T, how much work do the horses perform together over one full revolution of the drill?

    I got as far as drawing out the picture of this problem and starting the work: First of all the Work of one revolution is equal to 2*pi*torque right? Then in trying to find the torque... this is where I get confused.

    2. How much work do the biceps perform when lifting a 2 kg mass from the lower arm positioned 45 degrees downward to horizontal?

    This problem I dont even know where to start.

    Both of these problems involve torque and work but i am not sure how to relate them.

    Any help would be appreciated!!!!
  2. jcsd
  3. Oct 7, 2004 #2

    Doc Al

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    Staff: Mentor

    Welcome to PF!
    The torque exerted by a force about an axis is [itex]\tau = \vec{r}\times\vec{F} = r F sin\theta[/itex], where r is the distance from the axis to the point where the force is applied and [itex]\theta[/itex] is the angle the force makes with the radial vector.

    Since the work is done against gravity, start by finding the vertical distance the mass is lifted.
  4. Oct 8, 2004 #3


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    Minor point, Al, but you have equated a vector to scalars. Obviously it's not what you meant.
  5. Oct 8, 2004 #4

    Doc Al

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    Staff: Mentor

    You caught me being lazy, Tide! :blushing:
    Yes, the torque is a vector. [itex]r F sin\theta[/itex] is just the magnitude of the torque.
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