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The turning effect, what is actually physically happening that causes it?

  1. Sep 9, 2009 #1
    Ok, so if an obect is picked up by holding it at the end of a 5 metre stick, then the object requires more force to lift, than at the end of a 1 metre stick, or with no stick at all.

    1)Now I understand the Effect of this, What I don't understand is why this happens?
    The true mass of the object remains the same, but to me trying to lift it on a long stick it has a 'relative mass' which is far greater than its true mass.
    For the purposes of me lifting it, the object has gained a 'relative mass'.
    How can this be possible? What is physically taking place that causes this?
    It sound alot like getting something for nothing, and thats why i'm confused.

    2)Does this 'relative mass' work for centrifugal force aswell?
    If a 10kg weight upon the end of a fixed arm 5 metres long were to spin at 1 rotation per second, how much force does the 10 kg weight contain, compared to the same apparatus but with a 3 metre arm?
    Is the difference in force, all explained by the larger circumference (thus greater acceleration)that the 5m arm must travel in the 1 second?
    Or would turning effect play apart aswell?
    As ever, a bit long winded, but i would really appreciate some feedback, thankyou.
  2. jcsd
  3. Sep 9, 2009 #2


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    With a long stick only the contact forces at different areas of your hand are greater, but they are opposing each other, so the net vertical force exerted by the hand on the stick is the same. The stick acts as a lever and creates a large torque, while your hand has the shorter end:
    Last edited: Sep 9, 2009
  4. Sep 9, 2009 #3
    You have to do the same amount of work no matter how you lift it (ideally). You can either do that work quickly or take your time. Doing it quickly will tire you out more (it takes more force).

    Think about hiking up a mountain... You could bee line straight up to the top but that takes more force and tires you out. What you do instead is take your time and snake around the mountain.
  5. Sep 9, 2009 #4
    This is an example of a point mass Moment of inertia. This describes your 'relative mass'.

    The vertical force required to lift the mass remains the same, the torque applied due to the increased moment is what makes it more difficult to lift.

    Also to answer your second question this does affect rotation. Think of an ice skater spinning round, when they pull their arms in they spin faster.
    Last edited: Sep 9, 2009
  6. Sep 10, 2009 #5
    In the first case you will have to apply an extra moment equal to that created by the mass acting along the length of the stick inorder to lift the mass. It is this moment created by the mass that gives you the feel of an extra load or effort in lifting.
    Simply applying a weight equal to the hanging mass, at thelifting end wont help.

    For thr second case , Chris has done all the expalining right by the example.
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