Atwood machine -- Question about the pulley and weights

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Discussion Overview

The discussion revolves around the mechanics of an Atwood machine, specifically focusing on the roles of tension, gravity, and the pulley in the system. Participants explore the forces acting on the masses and the implications of tension in the rope.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the tension in the rope acts upwards on the masses, while gravity acts downwards, determining the direction of acceleration based on which force is greater.
  • Others argue that the pulley primarily changes the direction of the tension force without applying force directly to the masses.
  • A participant notes that tension is a condition in the rope and does not have a directional nature, but rather acts vertically at the ends of the rope.
  • There is a discussion about the representation of tension in diagrams, with some participants questioning the labeling and clarity of the diagrams presented.
  • Some participants emphasize the need for clarity in discussing tension, suggesting that simply stating "tension" is insufficient without context.

Areas of Agreement / Disagreement

Participants express differing views on the nature of tension and its representation in diagrams, indicating that there is no consensus on these aspects. The discussion remains unresolved regarding the clarity and correctness of the diagrams and the conceptual understanding of tension.

Contextual Notes

Limitations include potential misunderstandings of the role of the pulley and the representation of forces in diagrams, as well as the ambiguity in the term "tension" without further specification.

rudransh verma
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I want to ask in this machine when there are two masses hanging down then the tension T is directed upwards along the rope. Is it the force applied by the rope on the mass? Is it the force applied by the pulley? When the anyone mass of the machine moves downwards is it because the force of gravity >tension T ?
 

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Assume an ideal massless unstrechable rope and an ideal frictionless pulley. Assume that the pulley is fastened to the ceiling or otherwise secured because you did say “Atwood machine”

The rope applies an upwards force on the mass and gravity applies a downwards force. Depending on which is greater the mass will accelerate upwards or downward.

The pulley is applying a force on the rope, not the masses.
 
A rope under tension exerts a force at each end. So the rope pulls up on the mass and down on the pulley. And if the downward force of gravity is greater than the tension pulling up on a mass, that mass will accelerate downward. (And vice versa.)

Just apply Newton's 2nd law to each mass.

(Oops... Nug beat me to it.)
 
  • Informative
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The only function of the pulley is to change direction of the force or tension in the rope or string keeping those two bodies linked together and behaving as one.
It can change direction several times.

1643213099572.jpeg

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/atwd.html

When the system moves, the force of gravity could be equal to (mass moves at constant velocity) or greater than (mass accelerates) the string tension T.
 
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rudransh verma said:
then the tension T is directed upwards along the rope
Strictly speaking, tension is neither "upwards" nor "downwards". It is vertical. It is a condition in the rope. A third law force pair between each and every incremental section of rope and the next. A force pair does not point in one direction or the other. It points in both.

At the ends of the cord, the tension can act in only one of those two directions. It pulls upward on the connected object at the bottom and downward on the connected object at the top.
 
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Nugatory said:
The pulley is applying a force on the rope, not the masses.
The gravity is applying the force on mass. Rope is applying force on mass in upward direction. Similarly, rope is applying force on pulley in downward direction and pulley is to the rope in upward direction. Got it!
Thanks @Doc Al
 
Lnewqban said:
The only function of the pulley is to change direction of the force or tension in the rope or string keeping those two bodies linked together and behaving as one.
Is this diagram correct. Is there a T in downward direction too?
 

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rudransh verma said:
Is this diagram correct. Is there a T in downward direction too?
You have ##T_I## written twice on that drawing. What are either of them supposed to be labelling?
 
jbriggs444 said:
You have ##T_I## written twice on that drawing. What are either of them supposed to be labelling?
Tension?
 
  • #10
rudransh verma said:
Tension?
Tension as a condition in the two segments of cord? That would be fine.

Tension as a supporting force applied to the weights? That would also be fine. Though it could then appear four times - once on each weight and twice on the pulley.

Just the word "tension" by itself is unenlightening. Use your words and say what you mean.
 
  • #11
rudransh verma said:
Is this diagram correct. Is there a T in downward direction too?
What are those green arrows representing?
 
  • #12
jbriggs444 said:
Just the word "tension" by itself is unenlightening. Use your words and say what you mean.
So there are four Tensions in this machine, all T. Can be more in complex machines!
Lnewqban said:
What are those green arrows representing?
I don't know
 
  • #13
rudransh verma said:
I don't know
If you do not know what a drawing means then the question of its correctness does not arise.
 

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