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Two men want to carry a wooden beam weighing 200

  1. Nov 15, 2009 #1
    Not sure about this one, any help?

    Two men want to carry a wooden beam weighing 200 lbs which is 20 feet long. Both men are equal height however one is stronger than the other and wishes to bear 50% more the weight. How far from the end of the beam should both of the men be to make this possible?
     
  2. jcsd
  3. Nov 15, 2009 #2

    cepheid

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    Re: Lifting

    Hi NoBodyKnows, welcome to PF!

    For future reference, this should be in the Homework Help subforum and should be posted using the template for homework problems. Hopefully a moderator will move it.

    What have you done so far for this problem? Are you familiar with the concept of torque? Also, you can easily work out the ratio between the force exerted by the stronger man and that exerted by the weaker man, right?
     
  4. Nov 15, 2009 #3
    Re: Lifting

    I understand the idea of using torques. We're trying to seperate the lifts into two different problems. Main problem being that the distance we'd use in any formula is relative to what? the center of the beam?
     
  5. Nov 15, 2009 #4

    cepheid

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    Re: Lifting

    Yeah, relative to the point around which the torques would be exerted. It follows just from the definition of torque that if you're trying to prevent a net rotational acceleration about the centre of mass of the beam, you'd balance the torques about that point, meaning that all of your lever arms would be measured relative to that point.
     
  6. Nov 15, 2009 #5
    Re: Lifting

    For static problems, in general you can do the following:

    Write down the equation that total force is zero (of course take forces as vector with direction, i.e. consider the sign)

    Write down total torque about any single point of your choice and equate total torque to zero. Make sure that you take the force component perpendicular to the line to your "center point".

    Only for non-stationary problems the torque from external forces and moment of inertia should be taken about the center of mass.

    Ah... however, don't forget the gravitational force if you don't use the center of mass as your "center torque point".
     
  7. Nov 17, 2009 #6
    Re: Lifting

    I would not refer to torque at all when solving this problem, or moments of inertia. It is a simple matter of segmentation. One fellow wants to lift 50% more than the other. So use ratios. Break it down to simple whole numbers. One fellow carries 2 apples - the stronger fella carries 3 apples - that makes a total of 5 apples. It is easy to mentally divide the beam into 5 equal parts and then decide where the two people should be stationed.
     
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