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Homework Help: Energy calculations

  1. May 4, 2014 #1
    Hi, I'm designing a gadget whereby I'm spinning a plastic propeller attached to a shaft which is attached to a 6V DC motor.

    I'm required to do a basic energy analysis of the set-up. The kinetic energy of the propeller is 0.5*I*w^2 (w=omega=angular velocity).

    Due to friction (air etc) then the motor will presumably have to supply energy to 'replenish' this lost energy.

    My questions are:

    1. Is the energy supplied by the motor constant? Or is there an initial surge to get the propeller going, then a somewhat lesser energy supply to keep it going?

    2. How do I relate this to time; i.e. how do I analysis power usage?

    Any ideas???
  2. jcsd
  3. May 4, 2014 #2


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    Staff: Mentor

    This is energy per time, or power.
    This is hard to tell and depends on details of the motor.
    What exactly do you want to analyze?
  4. May 4, 2014 #3
    Hi, Ok, If you can answer this then it'll help me figure this out. Presuming no energy is lost between the motor and spinning propeller, then the propeller has a kinetic energy = energy supplied by the motor?

    If the propeller has this kinetic energy for say 60 secs then what is the power usage of the propeller?
  5. May 4, 2014 #4


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    Gold Member

    Yes. And that is because of the law of conservation of energy.

    P is power
    E is energy
    t is time
  6. May 4, 2014 #5


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    Staff: Mentor

    The spinning propeller itself will lose energy.

    Whatever the propeller will lose to the environment, and this is the same as the motor has to provide to keep it spinning.
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