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

[Paulo Serrano]

Homework Statement

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?

Homework Equations

P=VI
Potential energy=mgh

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.

 

[Phyisab****]

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.

 

[Phyisab****]

Ok I don't know how you got 420 kJ but if you use that then everything works out.

 

[Paulo Serrano]

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! :)

 

[Phyisab****]

Newton's Second Law, F=ma.