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Forces and Torques

  1. Oct 14, 2004 #1
    I can't understand why I can't get this problem right. It's irritating me, so any help would be greatly appreciated.

    The Problem: The drawing shows a jet engine suspended beneath the wing of an airplane. The weight W of the engine is 9300 N and acts as shown in the drawing. The rotational axis in the drawing is perpendicular to the plane of the paper. With respect to this axis, find the magnitude of the torque due to the following force: the weight.


    I know how to find torque using Torque = Fl

    Initially my equation consisted of:

    Torque = 9300(2.50)cos 32 - But my answer was wrong. So I then thought perhaps I should being using gravity in there since W = mg. In using that my second equation consisted of:

    Torque = 91140(2.50)cos 32 - But again my answer was wrong.

    What am I missing?? This should not be that difficult that's why I'm getting frustrated that I can't figure out something simple, that I'm not just doing right.
  2. jcsd
  3. Oct 14, 2004 #2


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    Try dividing by cos32 instead of multiplying. Not promising anything, just saying give it a try.

    I seem to remember doing problems like this in the past and that was the answer.
  4. Oct 14, 2004 #3


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    Homework Helper

    You are having problem with geometry, look at the triangle again!
  5. Oct 14, 2004 #4

    Doc Al

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    Staff: Mentor

    Why do you multiply by cos 32 ?
    The weight is given; no need to calculate it.
  6. Oct 15, 2004 #5
    I have no idea why I multiply by cos 32. I was just following an example in the book that was similar. Is it supposed to be sin?
  7. Oct 15, 2004 #6

    Doc Al

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    Staff: Mentor

    You tell me. :smile: What's the definition of torque? What distance is involved?
  8. Oct 15, 2004 #7
    Torque is the magnitude of the force times the lever arm. The lever arm being the distance that is invloved.

    So.....I still don't know what I'm doing wrong.
  9. Oct 15, 2004 #8

    Doc Al

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    Staff: Mentor

    The lever arm is the perpendicular distance from the axis of rotation to the line of action of the force.

    As Cyclovenom noted, you are having a geometry/trig problem. (Examine the triangle involved, find the side that represents the lever arm, and determine which trig function you need.)
  10. Oct 15, 2004 #9
    Ok, I'll give it another go. Thank you for your help. It's much appreciated. :)
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