Finding the power of an elevator motor

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Homework Help Overview

The problem involves calculating the average power of an elevator motor during its acceleration phase and comparing it to the power at cruising speed. The subject area includes dynamics and power calculations in physics.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to calculate average power using a formula involving force and distance but expresses confusion about the transition to the cruising speed scenario. Participants suggest using different approaches, including considering accelerated motion and average velocity.

Discussion Status

Participants are actively discussing the calculations and clarifying the differences between the two parts of the problem. Some guidance has been offered regarding the use of appropriate formulas for both parts, but no consensus has been reached on the final calculations.

Contextual Notes

The original poster is uncertain about the assumptions made in their calculations and how to apply them to the second part of the problem. There is an indication of homework constraints that may limit the methods available for solving the problem.

petefic
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Homework Statement



A 650kg elevator starts from rest. It moves upward for 3s with constant acceleration until it reaches its cruising speed of 1.75m/s.
A) What is the average power of the elevator motor during this time period?
B) How does this power compare with the motor power when the elevator is at its cruising speed?

Homework Equations


I think I did part a correctly, but I would like someone to check it. I don't really understand where to start with part b, how is it different than part a?


The Attempt at a Solution


Pavg = W/dT = (F*d)/t = ((ma+mg)*(v*t))/t = ((m(v/t)+mg)*(v*t))/t = (650kg(((1.75m/s)/3s) + 9.8m/s^2)*(1.75m/s*3s))/3s = 11811.04W
 
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Welcome to PF, Pete.
All okay except the replacement of d with vt. That would be assuming the motion is at constant speed, which it is not. Use an accelerated motion formula to find d.
The formula P = F*v will also work with the average velocity. Should be half your answer.
 
Thanks. Do you have any idea on how to get started on part b?
 
Same thing but leave out the ma term, which is zero.
Velocity is steady on 1.75 now, so you could use d = vt.
 

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