Effect of friction on the tension in a pulley

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Discussion Overview

The discussion revolves around the effect of friction on the tension in a pulley system with two masses. Participants explore how the presence of friction alters the tension in the string when one mass is pulled down and released, leading to acceleration in the system. The conversation includes considerations of different friction scenarios and their implications on the forces involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes a scenario where tension in the string is analyzed without friction and then considers the implications of friction on both sides of the pulley, leading to confusion about how tension can change.
  • Another participant questions whether the string slides over the surface of the pulley and whether the pulley has mass or friction in its bearings, suggesting these factors are crucial for analysis.
  • Some participants assert that if the pulley rotates, friction must be considered at the axle rather than at the string directly, which could affect the analysis.
  • There is a discussion about the assumption that tensions on both sides of the pulley must be equal, with one participant noting that this is only true if there is no friction and the pulley is massless.
  • A later reply indicates that it is possible for tension to increase on one side and decrease on the other, depending on the friction present.

Areas of Agreement / Disagreement

Participants express differing views on how friction affects tension in the pulley system. There is no consensus on whether the tensions must remain equal or how friction should be modeled in this context.

Contextual Notes

Participants have not fully resolved the implications of friction on the tension in the system, and there are assumptions about the nature of the pulley and the friction forces that remain unclarified.

rasen58
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I measure the tension in a pulley system with two masses (one smaller, one larger) where I pull the small mass down and let it go so that the system accelerates in the direction of the larger mass. This is without friction.

Then, I try to consider friction. Does the tension in the string change?

How I tried to think about it was on either side of the pulley. So on the side with the smaller mass, which goes upward after letting go, if there is friction, then the friction force points downward and adds with the gravitational force. So therefore, I think that the tension force pointing upwards has to increase as well to balance the new friction force pointing downward.
But if you look on the other side, with the large mass, you see that it is going down. So the gravitational force points down and the tension force points up. The new friction force also points up, but because the gravitational force can't change, I think that the tension force would have to decrease to match the increase in friction force upwards, so that the net force stays the same.

But the tension can't increase on one side and decrease on the other, so I'm confused. Did I think about it in the wrong way?
 
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Does the string slide over the surface of the pulley? Does the pulley have mass? Is there friction in the bearings of the pulley?

Chet
 
I would think the string slides over the surface of the pulley?
Massless pulley
There is friction everywhere on the pulley that there can be friction, I don't think it would matter where in order to analyze the problem
 
rasen58 said:
I would think the string slides over the surface of the pulley?
Really? Usually a pulley is meant to rotate, so the string doesn't have to slide.
rasen58 said:
I don't think it would matter where in order to analyze the problem
If you want to analyze it quantitatively, then it does matter if the friction force is at the pulley axle (no sliding) or the string directly (sliding).
 
Oh right, well the pulley actually does rotate, my fault.

Then I guess the friction is at the pulley axl
 
rasen58 said:
But the tension can't increase on one side and decrease on the other

Why not? If you're assuming that the tensions on the two sides have to be equal, this is true only if there is no friction in the pulley and the pulley is massless. To see this, apply the rotational form of Newton's Second Law to the pulley: $$\sum \tau = I \alpha$$
 
Last edited:
@jtbell So I was right? It actually does increase on one and decrease on the other? Thanks
 

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