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Homework Help: Mass Needed to Keep Rod Spinning at 1200RPM

  1. Mar 13, 2009 #1
    1. The problem statement, all variables and given/known data

    I have three metal blocks evenly spaced in a 0.3m circle that attach to a rod in the center of the circle. The rod requires 15HP to keep rotating at 1200RPM. What would the mass of the three blocks spinning at 1200RPMs need to be in order to produce 15HP? My physics class hasn't got to anything that would help me figure this out, so I don't even know where to start. I need this for a something I'm trying to build not for school.
    2. Relevant equations

    3. The attempt at a solution
    Last edited: Mar 13, 2009
  2. jcsd
  3. Mar 13, 2009 #2
    Re: Need to know mass required to keep a rod spinning.

    Since power is related to torque and rotation rate, you can find the torque required to keep the rod spinning. Generally, the equation used to relate torque to rotation is:


    What you're asking is how the geometry and mass of these blocks affects the inertia of the system (I, in the above equation). However, in your case [tex]\alpha[/tex] is zero because you're at a constant speed, which means you don't need any torque (or power) to maintain it. The equation you're looking for probably looks something like:




    or a combination/variation of the two of them, where b and c are related to the viscous damping coulomb friction in the system.

    So basically what I am trying to say, is that you don't have enough information. If you're using 15 HP to maintain, then you have some kind of friction in your system, and this power is being used to overcome those losses, not to drive your system's inertia. In fact, those masses should be independent of the power consumed at a constant speed.

    I feel like I did a lousy job explaining this... was I clear enough?

  4. Mar 13, 2009 #3
    I just answered this same post (by you...) in another forum. Isn't that a no-no?
    Last edited by a moderator: Mar 13, 2009
  5. Mar 13, 2009 #4


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    Staff: Mentor

    I merged the two threads. Yes, multiple posting of threads is not allowed here on the PF.
  6. Mar 13, 2009 #5
    Sorry about that, I didn't know which place to post it so I did it in both. What I'm trying to figure out is I have a generator and they told me it takes 15HP to keep the generator turning while it is under rated load. I just wanted to know if the masses are large enough and spinning fast enough, would it keep the generator spinning and still producing energy. So what other info would I need, or is it even possible?
  7. Mar 14, 2009 #6
    If you have system that is creating energy, and it is not loosing any energy (it's not slowing down) and there's no energy being put into the system (no fuel and combustion, for example), then you're creating "free" energy. You could solve the world's energy problems...

    Unfortunately, this is not possible. Energy must be conserved (Google 'First Law of Thermodynamics' or 'Law of Conservation of Energy'). If energy is leaving your system (via electrical current, in your case), then the system will loose energy (slow down) or energy must be put back into the system.

  8. Mar 14, 2009 #7


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    Staff: Mentor

    The masses have nothing to do with the question, unless the torque input power is pulsed. Then the masses can act as a flywheel to smooth the energy transfer. The torque input on the shaft of your generator is what results in the electrical power output.
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