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Lifting boxes of different sizes

  1. Apr 8, 2015 #1
    Okay so 8n college I majored in math but throughout my education never had a real physics course. Now I currently have a labor j9b lifting boxes. At the warehouse we separate heavier boxes based solely on wieght and not size. So I was wondering does (assuming wieght is equally distributed) does it take more force to lift a larger box since the wieght would be further away from you?
  2. jcsd
  3. Apr 8, 2015 #2
    No, it does not take more force. Force (as matters to this question) only involves the mass and the gravitational attraction to the Earth (acceleration due to gravity). Yes, there are a ton of forces actually acting on the box (ie friction, electromagnetic, etc), but the only one that matters here is gravitational force. It may require greater torque, which is equal to the force perpendicular times radius of the moment arm. This requires more mechanical energy, or more work.
  4. Apr 8, 2015 #3


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    Staff: Mentor

    The main issue is the increased torque you need to generate for a given weight that is held farther away from your body. That puts stress on some of your muscles, which does require more calories to be burned. It also places your back's health more at risk, the larger and larger the boxes get for a given weight. At some point you will need two people to make it a safe lift.
  5. Apr 8, 2015 #4


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    Staff: Mentor

    I would answer yes. Post #2 was about the force applied to the box to counter gravity, but I don't think that's the force the OP is interested in: I think the OP is interested in the force(s) exerted by his muscles. And a physically larger object does absolutely require you to generate more force to lift it if you have to hold it further away from your body due to its bulk. That additional force is from the torque(s) involved. Indeed, if you use leverage properly, you can in some instances decrease the forces applied by your muscles to zero or near zero (consider, standing).

    Consider if you hold a small object at arms-length away from you. Say your arm is 2' long and the object weighs 1 lb. That's 2 ft-lb of torque applied even if you exclude the weight of your arm. If the knob of your shoulder that provides the fulcrum and leverage for muscle action is 2" in diameter, that's 12 lb of force applied.

    Now bend your elbow and hold the object half as far away. Now the torque at your shoulder is cut in half and the force applied by your deltoid muscle is therefore cut in half (an additional force is now applied by your bicep).

    Now place the object on your shoulder. How much force does your deltoid apply to your shoulder now?

    Lift with leverage. If you've ever wrestled or taken judo, you know that the throws tend to be less impressive than they look because they take advantage of leverage and requrie surprisingly little strength.
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