Pulley Small or Large for a Motor

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    Motor Pulley
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Homework Help Overview

The discussion revolves around the mechanics of a motor and pulley system, specifically addressing the relationship between the diameter of the pulley and the motor's ability to lift a load, such as a pail of water. Participants explore whether increasing or decreasing the diameter of the pulley would enable the motor to lift the load effectively.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants consider the implications of pulley diameter on torque and lifting capacity, with some suggesting that increasing the diameter may help, while others argue for a reduction based on constant torque. Questions arise regarding the setup of the motor and pulley, particularly the attachment of the rope and its effect on tension and torque.

Discussion Status

The discussion is active, with participants sharing differing viewpoints and questioning their assumptions. Some have offered insights into the relationship between torque and pulley diameter, while others express uncertainty about their understanding of the problem setup.

Contextual Notes

There is a lack of clarity regarding the configuration of the motor and pulley system, particularly whether the rope is attached to the motor or just to the pulley. This ambiguity affects the participants' reasoning about the forces involved.

surajzakaas
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A motor can't lift a pale of water for given diameter of pulley. (consider the motor directly connected to the pulley). So, the diameter of the pulley should be Increased or decreased? Why?
 
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welcome to pf!

hi surajzakaas! welcome to pf! :wink:

tell us what you think, and then we'll comment! :smile:
 
i thought the dia. should increase. but a friend said the dia should be reduced as the torque is constant. but i am unable to digest it.
 
hi surajzakaas! :smile:
surajzakaas said:
A motor can't lift a pale of water …
surajzakaas said:
… a friend said the dia should be reduced as the torque is constant …

the torque from the motor is constant, but that isn't the same as the torque on the pulley …

it's easier if we say that the maximum tension in the rope is fixed …

what is your opinion then, and why?? :wink:
 
Last edited:
if max tension is fixed then reducing dia will reduce the torque on pulley and vice versa. and as motor can't lift up the mass the torque given by motor is less. so i guess reducing the pulley dia will work. but how will that work practically i don't know.
still the first solution that comes to my mind is to increase the dia. I don't know why!
 
reading the question again, I'm not sure I've understood it correctly :confused:

is the other end of the rope holding the pail attached to the motor,

or is the rope just attached to the pulley, with the motor directly turning the axle of the pulley?

(if it's the latter, my previous comment about the tension in the rope was wrong :redface:)
 
the rope is just attached to the pulley, with the motor directly turning the axle of the pulley.
 
ah, so the driving torque τ on the pulley is constant :redface:

ok, the force mg needed to lift the pail is the the same as the force from the rope on the pulley

if the initial radius is r, then the inability of the motor to raise the pail can be written as the inequality τ < … ?

and so r needs to be …creased ? :wink:​
 
decreased.
 
  • #10
yup! :biggrin:
 

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