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Finding how high a winch can lift a mass before the battery discharges

  1. Mar 8, 2009 #1
    Finding how high a "winch" can lift a mass before the battery discharges

    1. The problem statement, all variables and given/known data

    The EMF of the D cell battery is 1.5 V and the life is about 10 A/hr. 50% of the energy int he winch does actual work and the lifting mass is 50 kg. The winch has a load resistance variable of Rl.

    2. Relevant equations
    Total Power=VI-V^2/Rl

    (1.5*10)-10^2/Rl Total power of the system since the question states that the winch's resistance is much greater than the battery's internal resistance. (Thus ignore battery's resistance.)


    3. The attempt at a solution


    After finding the total power of the system, I thought that convert the power into work and using mgh to find the max height of winch would work. But at this point, I do not understand how to convert power into work if I do not know the time. Am I to find the height in terms of hours? I've read the book and searched around the internet but the conversion between power and energy is confusing.


    Thank You
     
  2. jcsd
  3. Mar 8, 2009 #2

    mgb_phys

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    Re: Finding how high a "winch" can lift a mass before the battery discharges

    You are trying to fin the total ENERGY of the system.
    power = V * I and energy = power * time = V * I * t

    The capacity of the battery if 10 AHour (I*t) - you have written it down wrong.
     
  4. Mar 8, 2009 #3
    Re: Finding how high a "winch" can lift a mass before the battery discharges

    O I see. That should help me quite a bit. Thank You
     
  5. Mar 8, 2009 #4
    Re: Finding how high a "winch" can lift a mass before the battery discharges

    also, P is VI but don't I need to take into account the resistance of the winch it's self? Or does the 50% efficiency take into account of the resistance?
     
  6. Mar 8, 2009 #5

    mgb_phys

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    Re: Finding how high a "winch" can lift a mass before the battery discharges

    Yes the 50% efficency includes everything - the question is much simpler than you are imagining
     
  7. Mar 8, 2009 #6
    Re: Finding how high a "winch" can lift a mass before the battery discharges

    so in the next problem, we have to assume that the 50% doesn't apply. I now have to find the Rl which would maximize the energy. ..

    This is where I would apply the total energy-energy due to winch correct?

    My question here is that how do I find energy loss due to the winch? Can i use E=I^2Rt? But I'm confused since I is squared. can I assume that 10 A*hour is just I also? so that energy is 10^2*Rl*t and power is 10^2Rl?
     
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