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Force calculation help

  1. Jun 30, 2011 #1
    Hi all,

    I am building a spit to roast a pig. Instead of just getting a motor recommended to be powerful enough I would like to understand the math required to spec it out. I have no experience with physics but I have been reading about force, torque, and power.

    Most of what I have read about torque relates to the product of force exerted on a lever attached to an axis or fulcrum at a particular speed. But what if there is no lever?

    I want to know how much force is necessary to rotate a 100 lb pig. I understand there will be gravitational forces and frictional forces mitigating the inertia of the pig. Furthermore, I will be using gears to reduce the 1,000 rpm speed of the motor to 3-4 rpm for the spit.

    An explanation of the necessary math for this problem would be much appreciated.



    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 30, 2011 #2


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    Homework Helper

    Well first off, you can calculate the torque the motor will produce at the spit end by assuming no power losses so

    T1ω1 = T2ω2

    You have ω1 = 1000 rpm, ω2 = 3-4 rpm and the motor should have a torque rating T1 so T2 is easy to obtain.

    If you approximate the pig to a known shape such as a cylinder, you can measure an average radius of the pig 'r' and find its mass moment of inertia (I).

    Depending on how long you want the pig to go from 0 rpm to 3-4 rpm will depend on a time you put it as. You can get angular acceleration (α) from that information.

    Using I and α , you can obtain the torque T and see if that is comparable with T2.

    I tried to explain it as best as how I would try to do it seeing as how you have little to no experience with physics and you can look up the certain terms you don't know about which might give you some more insight on the design aspect of it.
  4. Jun 30, 2011 #3

    the likening the pig to a cylinder seems smart to me. I'll work on this tonight to digest and work on it.

  5. Jun 30, 2011 #4

    what I'd really like to know is the torque required to spin the pig and work backwards to figure what motor I need after the gearing. (I might be mistaken, but your advice tells me what the torque will be at the pig given a known motor.)

    I'm not so concerned about the acceleration because this is going to go on once at 7am and shut off at 7pm, (and I'm happy to give it a shove.) So operational force is what I'm looking for.

    I'll do some searching for power required to turn a 100lb cylinder of 15" diameter.

  6. Jul 1, 2011 #5


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    It's the same principle, you'd just need to read the post from the bottom go up

    In the 'worst' case scenario (in all scenarios I am ignoring friction, so your motor sizing will need to account for it), you would want the motor to make the pig go from 0 to 3-4 rpm in some time. So if you want it to happen in say 1 second, then you can get the angular acceleration and the torque. This will get you the torque at the motor using the formula I posted before. Then you can get a motor that will produce the torque at the given rpm.
  7. Jul 1, 2011 #6

    Thanks for the help.

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