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Homework Help: How many watts/how much intensity needed to power an elevator?

  1. Oct 10, 2009 #1
    1. The problem statement, all variables and given/known data
    An elevator, with people in it, has a mass of 1400 kgs. The motor is hooked up to the power grid that provides 220 volts of power. The elevator has to go up 30 meters at 4m/s.

    How many watts of potential (not sure if it's called that) are required to get the elevator moving and what is the maximum current passing through the motor?

    2. Relevant equations
    Potential energy=mgh

    3. The attempt at a solution

    Not sure. I know the answer but don't know how to get to it. My idea is to calculate the change in potential energy that occurs when moving up 30m. That gets me 420kJ. That's as far as I got.

    Answer: 56kW and 255A.
  2. jcsd
  3. Oct 10, 2009 #2
    I'm not sure this is right, but to continue what you started,

    PE = mgh = 411.6 kJ

    x=vt, t=x/v t=(30m)/(4m/s) = 7.5 s

    P = w/t = (411.4 kJ)/(7.5s)=54.9 kW

    which is kinda close to the answer. Then,

    I = P/V = (54.9 kW)/(220 V)=250 Amps

    which is also not quite the answer you posted, I dunno.
  4. Oct 10, 2009 #3
    Ok I don't know how you got 420 kJ but if you use that then everything works out.
  5. Oct 10, 2009 #4
    This question is from a national college entrance exam in Brazil. We're not allowed to use calculators, so I rounded g=9.81m/s to 10m/s.

    Can you enlighten me on that formula you used to find the time? (x=vt) Just a name or wikipedia link will suffice. :)

    edit: nevermind, I'm an idiot. That's the most basic formula in physics. Thanks again! :)
    Last edited: Oct 10, 2009
  6. Oct 10, 2009 #5
    Newton's Second Law, F=ma.
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