# A1 mechanic question will there always be 2 tensions on eg a string pulling a mass t

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Thread starter 1 month ago
#1
will there always be 2 tensions on eg a string pulling a mass tension up and below?
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1 month ago
#2
(Original post by interlanken-fall)
will there always be 2 tensions on eg a string pulling a mass tension up and below?
Can you upload an actual question which illustrates what you're asking?

If Im right, you're asking about a string connecting two objects which will generally act to pull one object and drag the other object via the tension. In a sense, the string is just transferring the force between the two objects, so one object would drag / pull the other in the same way if they were directly connected.

Instead of considering a single system composed of the two masses, the tension allows you to break it up into two parts. When you add the seperate equations of motion together, the +/-tension cancels and you end up with the equation of motion of the composite system.
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#3
Why is there 2 forces of tension? Should there only be one and the one pulling the mass
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#4
Why is there 2 forces of tension one up and one down, shouldn’t there only one pulling the mass
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1 month ago
#5
(Original post by interlanken-fall)
Why is there 2 forces of tension? Should there only be one and the one pulling the mass
As per the previous post, the "two" tensions are equal and opposite. If you consider P, then Q is dragging P through the tension. If you consider Q, then P is pulling Q through the tension. If you add the equations of motion together, you get the motion of the composite system PQ with no tension being represented.
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1 month ago
#6
(Original post by interlanken-fall)
Why is there 2 forces of tension one up and one down, shouldn’t there only one pulling the mass
Similar to the other example, the tension drags one mass (slows down the descent) and the tension pulls up the other mass (beats gravity) with equal magnitude, opposite direction. Adding them together gives the motion of the composite system.
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#7
Will there always be 2 tensions? In this example they found the tension going up, will the tension going down be the same ?
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1 month ago
#8
(Original post by interlanken-fall)
Will there always be 2 tensions? In this example they found the tension going up, will the tension going down be the same ?
Can you make it a bit easier to read pls ... orientation, size, which part ...

If its a "normal" string, under tension then yes. Newton 3 basically applies.
https://physics.stackexchange.com/qu...tons-third-law
(probably better examples if you google).
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#9
(Original post by mqb2766)
As per the previous post, the "two" tensions are equal and opposite. If you consider P, then Q is dragging P through the tension. If you consider Q, then P is pulling Q through the tension. If you add the equations of motion together, you get the motion of the composite system PQ with no tension being represented.
(Original post by mqb2766)
Can you make it a bit easier to read pls ... orientation, size, which part ...

If its a "normal" string, under tension then yes. Newton 3 basically applies.
https://physics.stackexchange.com/qu...tons-third-law
(probably better examples if you google).
still a bit confused, a the string if facing down the tension is the reason why it still up, wouldn't there be a equal and opposite force on the string so it pushs the masses equally down
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1 month ago
#10
(Original post by interlanken-fall)
still a bit confused, a the string if facing down the tension is the reason why it still up, wouldn't there be a equal and opposite force on the string so it pushs the masses equally down
Can you draw the scenario you're considering and upload?
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#11
"find the tension in the string" but shouldn't there be a equal and opposite force on the string so it doesn't go up?
Last edited by interlanken-fall; 1 month ago
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1 month ago
#12
The tension is pulling up the "object" which consists of the pan and two masses. There must be an equal and opposite tension pulling down the *** (roof?) which is not on your diagram. If the string was not there, the roof(?) would pull up the pan/masses and the pan/masses would pull down the roof(?).
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#13
diagram in the book
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1 month ago
#14
(Original post by interlanken-fall)
diagram in the book
Do you understand the fact that the string effectively just transfers force (the tension), so there is an equal but opposite force applied to the objects at each end. This is why you have +/-T. If the string was not there (the objects were hard coupled), so the objects acted directly on each other, the behaviour would be the "same".
Last edited by mqb2766; 1 month ago
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1 month ago
#15
You are getting mixed up about the objects that the forces are acting on.

First make sure you understand Tension. The best explanations I found were here:

https://physics.stackexchange.com/a/307840/182137
https://physics.stackexchange.com/a/340573/182137

If that still does not make sense, some people describe Tension as a bit like a "3D pressure":
https://physics.stackexchange.com/a/567808/182137

If you can kind of get your head around that, then you need to follow mqb2766's advice and then think about Newton's 3rd laws.

Have a look at this first diagram:

From here:
https://deutsch.physics.ucsc.edu/6A/...ces/node7.html

First you need to consider the forces acting on the mass. This is labelled in red on the diagram. There will be the force due to gravity which goes down (directed towards the centre of the Earth). This is labelled . Now the mass is applying a downwards force on the rope. By Newton's Third Law, we therefore have an equal but opposite upward force, labelled applied on the mass.

Now consider the forces acting on the rope. This is labelled in blue. The rope is under tension due to the mass. So at the bottom, there is a force acting on the rope which goes downwards, labelled . At the top of the rope the hand (or it could be a pulley), is applying a force upwards to prevent the rope from simply falling to the floor. This is labelled .

Now consider the force acting ON the hand (pulley). The hand (pulley) is applying an upwards force ON the string. So the string applies an equal but opposite force on the pulley. It applies a force ON the pulley which acts down. This is labelled in green as .

Unfortunately, a lot of diagrams do not often show the forces in blue (acting ON the rope). They only show the forces in red and green, i.e. the forces acting on the mass and the hand (pulley) respectively. This is fine because there is an assumption that the force throughout the rope is uniform but the side effect is that it leads to the confusion that you have.
Last edited by 0le; 1 month ago
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1 month ago
#16
(Original post by 0le)
You are getting mixed up about the objects that the forces are acting on.

First make sure you understand Tension. The best explanations I found were here:

https://physics.stackexchange.com/a/307840/182137
https://physics.stackexchange.com/a/340573/182137

If that still does not make sense, some people describe Tension as a bit like a "3D pressure":
https://physics.stackexchange.com/a/567808/182137

If you can kind of get your head around that, then you need to follow mqb2766's advice and then think about Newton's 3rd laws.

Have a look at this first diagram:

From here:
https://deutsch.physics.ucsc.edu/6A/...ces/node7.html

First you need to consider the forces acting on the mass. This is labelled in red on the diagram. There will be the force due to gravity which goes down (directed towards the centre of the Earth). This is labelled . Now the mass is applying a downwards force on the rope. By Newton's Third Law, we therefore have an equal but opposite upward force, labelled applied on the mass.

Now consider the forces acting on the rope. This is labelled in blue. The rope is under tension due to the mass. So at the bottom, there is a force acting on the rope which goes downwards, labelled . At the top of the rope the hand (or it could be a pulley), is applying a force upwards to prevent the rope from simply falling to the floor. This is labelled .

Now consider the force acting ON the hand (pulley). The hand (pulley) is applying an upwards force ON the string. So the string applies an equal but opposite force on the pulley. It applies a force ON the pulley which acts down. This is labelled in green as .

Unfortunately, a lot of diagrams do not often show the forces in blue (acting ON the rope). They only show the forces in red and green, i.e. the forces acting on the mass and the hand (pulley) respectively. This is fine because there is an assumption that the force throughout the rope is uniform but the side effect is that it leads to the confusion that you have.
Nice explanation.
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Thread starter 1 month ago
#17
(Original post by 0le)
You are getting mixed up about the objects that the forces are acting on.

First make sure you understand Tension. The best explanations I found were here:

https://physics.stackexchange.com/a/307840/182137
https://physics.stackexchange.com/a/340573/182137

If that still does not make sense, some people describe Tension as a bit like a "3D pressure":
https://physics.stackexchange.com/a/567808/182137

If you can kind of get your head around that, then you need to follow mqb2766's advice and then think about Newton's 3rd laws.

Have a look at this first diagram:

From here:
https://deutsch.physics.ucsc.edu/6A/...ces/node7.html

First you need to consider the forces acting on the mass. This is labelled in red on the diagram. There will be the force due to gravity which goes down (directed towards the centre of the Earth). This is labelled . Now the mass is applying a downwards force on the rope. By Newton's Third Law, we therefore have an equal but opposite upward force, labelled applied on the mass.

Now consider the forces acting on the rope. This is labelled in blue. The rope is under tension due to the mass. So at the bottom, there is a force acting on the rope which goes downwards, labelled . At the top of the rope the hand (or it could be a pulley), is applying a force upwards to prevent the rope from simply falling to the floor. This is labelled .

Now consider the force acting ON the hand (pulley). The hand (pulley) is applying an upwards force ON the string. So the string applies an equal but opposite force on the pulley. It applies a force ON the pulley which acts down. This is labelled in green as .

Unfortunately, a lot of diagrams do not often show the forces in blue (acting ON the rope). They only show the forces in red and green, i.e. the forces acting on the mass and the hand (pulley) respectively. This is fine because there is an assumption that the force throughout the rope is uniform but the side effect is that it leads to the confusion that you have.
thank you for this! really appreciate it
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