Torque on a pulley with a mass

  • Thread starter hms.tech
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  • #1
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If there is a load of say, mass =m Kg which is being raised on the end of a light inextensible rope by means of a winch.

see attachment.

the pulley of the winch has a radium x meters and is lets say that it is frictionless.
the acceleration of the load is a ms^-2

can someone tell me how would i calculate the Torque / moment on the pulley ?
i know that torque = Force * Displacement

here is what i think shud be the correct equation :

net force =N = T - mg

N[x]= Torque

But the equation in my notes says otherwise :

let K = the torque applied by the winch (the torque driving the pulley)

Torque on the pulley = K - (x)(T)

which one is correct and why ?
 

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  • #2
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can someone tell me how would i calculate the Torque / moment on the pulley ?
i know that torque = Force * Displacement
The Force here is the force applied on the pulley. From the figure you shared, what is the force on the pulley?
 
  • #3
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The Force here is the force applied on the pulley. From the figure you shared, what is the force on the pulley?

i stated the Torque instead, and u have the radius (where the force is acting tangential)

the force on the pulley which causes it to rotate CLOCKwise = K/x
 
  • #4
185
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Where's K in your drawing?

my guess is that you mean K is some other given torque.
It should be in your drawing.

Torque depends on 2 things. The force and the lever arm.
so you need to keep track of what you're torque-ing
about. in this case the natural origin is the center of
the pulley.

Torques add up - just keep track of the sign. say we pick clockwise
as positive. and calculate torque (on the pulley) about the pivot

then the net torque is the sum of
torque due to K - which is just K
torque due to m_pulley g - which is 0 (since the lever arm is 0)
torque due to T - which is -xT (x is lever arm , and minus sign is down)
so net torque is τ=K - xT

------------
also the word is should not shud, it hurts my eyes to see the latter.
 
  • #5
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torque due to m_pulley g - which is 0 (since the lever arm is 0)
I dont understand this point .

Would you please explain it in more detail ? :confused:
 
  • #6
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Firstly, as qbert points out, we don't see 'K' in the image, so I cannot comment on it.

Secondly, my question exactly was "From the figure you shared, what is the force on the pulley?". The figure serves as a free body diagram of the pulley, so the force on the pulley is: T, the tension acting on the rim, downwards.

Now you can use your equation: torque = Force * Displacement

To calculate the tension force: Since you know the acceleration of the block and its mass, you can calculate the tension force. This tension force is the same as the one acting at the rim of the pulley.
 
  • #7
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This tension force is the same as the one acting at the rim of the pulley.

Why is that ?

i dont understand the free body diagram.

Why is TENSION of the string (applied on the pulley cuz the string is in contact with the pulley) causeing the Torque, why is weight not causing the torque ?
 
  • #8
466
1
I see no pulley in your picture. It looks to me like a simple hoist drum lifting a load.

If you add a frictionless Lilly to the rigging then the torque on it is zero. That is the whole point of putting good bearings in a pulley.
 
  • #9
1,233
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Why is TENSION of the string (applied on the pulley cuz the string is in contact with the pulley) causeing the Torque, why is weight not causing the torque ?
Because the weight itself is not directly attached or connected to the pulley.

The string tension is transferred to the pulley due to friction. If there were no friction, the string would slide over the pulley and produce no torque.
 
  • #10
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Because we are calculating torque about the center of mass of the pulley and weight of the pulley is acting at the center of mass.

There is no torque at a point due to a force at the same point. Do you know why?

EDIT: Above post answers why weight of the block is not causing torque, this post answers why weight of pulley isn't causing torque.
 
  • #11
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There is no torque at a point due to a force at the same point. Do you know why?
Because the weight of the pulley acts downwards passing through the axis of rotation, thus the Torque = W*r

W = weight of pulley
r= zero
 
  • #12
631
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Right, so weight is not causing the torque. Tension is applied at the rim of the pulley, it's not passing through the axis of rotation, so it does have a torque.
 
  • #13
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Another related problem for me [here]
 

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