# Homework Help: Elevator with Power

1. Nov 4, 2008

### Geminiforce

1. The problem statement, all variables and given/known data
A 500 kg elevator starts from rest. It moves upward for 3.50 s with constant acceleration until it reaches its cruising speed, 1.75 m/s.
(a) What is the average power of the elevator motor during this period?
(b) How does this power compare with the motor power when the elevator moves at its cruising speed?
Pcruising = ?
2. Relevant equations
F=ma
P=W/deltaT
W=Fdelta(r)cos(theta)

3. The attempt at a solution
This problem, I first found the acceleration using Vf= Vi +at
a= 0.5m/s^2

Next I plugged this into delta x = Vot +1/2a(t^2) and found delta x to be 3.0625
Next, I solved for tension using: T-Mg = Ma
T= M(a+g) = 10,578.75 W
Is this correct? If it isn't, can someone explain to me how to approach this problem?

2. Nov 5, 2008

### Hootenanny

Staff Emeritus
Your method looks good up until this point:
Perhaps this is a typo but, tension is a force and not power.

3. Nov 5, 2008

### HallsofIvy

Why? The problem did not ask for distance moved.

No, the units are wrong. kg (m/s2 is Newtons, not Watts. This is a force not power. Do you know how to convert from force to power, i.e. from Newtons to Watts?

4. Nov 5, 2008

### Geminiforce

You're right i accidently put the force in the wrong units....
I meant to put Power=M(a+g)(v)... However the question asks for the average power and I think the power calculated there is the power at 3 second. Would this power i just calculated be the cruising speed?