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Rotating heavy masses x100 of you're own?

  1. Sep 9, 2012 #1
    Hello everyone!

    A few days ago I was in my university I walked in my engineering building and I saw a HUDGE pendulum! Very big and so heavy! It was hung from the celling wall, not sure of its mass though... Anyhow! I was wondering why is it when we stabilize the mass of an object equally we as humans very weak and small in mass can rotate it easily!

    I never thought of it properly, but really if I was able to lift up a truck! and hang it from both ends top/bottom and make it semi-frictionless(extremely low friction) I can rotate it easily with not much effort!

    This idea came in mind when I saw a huge ball being rotated by a small motor x100 its size in a building!

    I mean is the motor only rotating it? Does it feel any resistance of the weight? Or that can be solved?


  2. jcsd
  3. Sep 9, 2012 #2
    You wouldn't actually be able to rotate the truck that easily. However, it's true that if you applied constant force to the hanging truck for some time, could rotate it. You mentioned the reason for this in your post: there's little to no friction resisting the motion of the truck. When you try to roll the truck across the ground, it'll be much harder because the kinetic energy you transfer to the truck is quickly taken back out of the truck by the friction in the wheels and axles of the truck. When the truck is suspended, there's much less friction, allowing the truck to better maintain the kinetic energy you're transferring to it.

    The motor for the pendulum doesn't need to be very big because it can constantly apply a small force which adds up to a large momentum in the pendulum over time, and if the pendulum doesn't produce much friction, it will maintain that momentum / kinetic energy for a long time.
  4. Sep 9, 2012 #3


    If you suspend a large mass and apply a small force over time eventually it will speed up and rotate and become easier to move :approve:

    In a sense if I lift up a very very very heavy object x100 of my own mass. If I applied some force it will eventually add up and move based on my force.

    Again a small motor can rotate a heavy mass that is x100 its size if suspend equally and has low mechanical friction as possibile!
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