# Atwood machine -- Question about the pulley and weights

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• rudransh verma
In summary, the conversation discusses the concept of tension in an Atwood machine with two masses hanging down and a pulley. The tension in the rope pulls up on the masses and down on the pulley, while gravity applies a downwards force on the masses. The direction of the tension depends on which force is greater and can change depending on the movement of the masses. The only function of the pulley is to change the direction of the tension in the rope. The diagram provided is incorrect as it does not accurately label the tension forces.
rudransh verma
Gold Member
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.)

rudransh verma
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.

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.

rudransh verma
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?

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.

rudransh verma said:
Is this diagram correct. Is there a T in downward direction too?
What are those green arrows representing?

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

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.

## What is an Atwood machine and how does it work?

An Atwood machine is a simple mechanical device that consists of a pulley, a string, and two weights. The weights are connected by the string, which passes over the pulley. The machine works by using the force of gravity to accelerate the weights in opposite directions. The heavier weight will accelerate downwards, pulling the lighter weight upwards, and the pulley helps to redirect the force.

## What is the purpose of the pulley in an Atwood machine?

The pulley in an Atwood machine serves to redirect the force of gravity. Without the pulley, the weights would simply fall to the ground. But with the pulley, the force of gravity is redirected, causing the weights to accelerate in opposite directions.

## How does the mass of the weights affect the motion of an Atwood machine?

The mass of the weights affects the motion of an Atwood machine by determining the amount of force that is applied to the system. The heavier weight will accelerate downwards with a greater force, causing the lighter weight to accelerate upwards with a smaller force. This results in a net acceleration of the system.

## What is the formula for calculating the acceleration of an Atwood machine?

The formula for calculating the acceleration of an Atwood machine is a = (m1 - m2)g / (m1 + m2), where m1 and m2 are the masses of the two weights and g is the acceleration due to gravity (9.8 m/s²). This formula assumes a frictionless system and a massless pulley.

## What factors can affect the accuracy of an Atwood machine's acceleration?

The accuracy of an Atwood machine's acceleration can be affected by factors such as friction in the pulley or string, air resistance, and the weight of the pulley itself. These factors can introduce additional forces into the system, making it more difficult to accurately measure the acceleration of the weights.

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