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Inertia and Torque

  1. Aug 3, 2010 #1
    All this recent talk of torque and horsepower leads me to show my ignorance and ask for enlightenment (I tried to post this yesterday but it seems I failed, sorry if its a dup):

    Say I want to spin a nicely balanced wheel. I can calculate the moment of inertia but can't quite grasp how that helps me figure out how big a motor I need. My problem is dimensional analysis:

    moment of inertia == kg·m²
    energy (joules) == kg·m²·s² (or Newton-Meter)
    torque(joules/radian) == kg·m²·s² (where wiki sez: "A torque of 1 N·m applied through a full revolution will require an energy of exactly 2π joules.")

    So... ignoring friction, the torque of the motor only influences the acceleration of the wheel? Then it is just friction that prevents me from driving my truck tire with a cell-phone vibrator motor? Or is there another conversion I must yet undergo?
  2. jcsd
  3. Aug 3, 2010 #2


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    That explains the torque required, but there's no stated period of time for that amount of energy to be added. Power is the rate of energy change per unit time. So if the 2n joules could be applied in one second at constant power, then the power would be 2n watts.
  4. Aug 3, 2010 #3


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    Yeah, pretty much. If you disregard friction and other resistance forces, you could spin anything with even a tiny motor. But it would take ages to spin up to any reasonable velocity, and there's a decent chance the motor would blow out before long. (Working against a large load puts a lot of stress on the internal parts of a small motor)

    Like rcgldr said, in practice you usually have to consider timing, e.g. how quickly you want to achieve a certain angular velocity, and that tells you what power your motor needs to provide.
  5. Aug 4, 2010 #4
    Excellent! Thank you both. But further clarification needed, as usual:

    In order for me to be able to decide that a certain motor will spin my erstwhile lumpen truck wheel I need to measure the friction that needs to be overcome. To do so would be pretty much equivalent to measuring a lever-arm from the outside circumference, say by hooking a spring scale to the wheel and pulling until something gives? Then I can work out how fast I want to get things going by adding torque above and beyond with the 2pi joules routine?

    I shoulda taken physics and mechanical engineering back when I had functioning brain cells...instead I wasted them on learning the C language. Thanks again.
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