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Q&A Why does tension change from 15 N to 17 N when forces are replaced by weights?

I'm working on a problem involving a pulley system, and I’m confused about how the tension changes when forces are replaced by masses. Initially, if I apply forces of 20 N and 15 N directly at the...

2 answers · posted 2mo ago by Thor ‭ · last activity 1mo ago by harmony‭

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#2: Post edited by

Thor ‭ · 2024-10-05T16:11:59Z (about 2 months ago)

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~~I'm working on a problem involving a pulley system, and I’m confused about how the tension changes when forces are replaced by masses.~~
Initially, if I apply forces of 20 N and 15 N directly at the ends of a massless string (without any pulley or masses), the tension in the string is exactly 15 N, as expected.
However, when I introduce masses (which exert 20 N and 15 N due to gravity) and place them over a pulley, the tension comes out to be 17 N instead of 15 N. I understand how to solve this mathematically, but why does the tension increase to 17 N when masses are used, and why does the pulley play such a crucial role?
I tried working through the mechanics, but I can't quite grasp the intuition behind the change in tension. I've attached an image of my attempt to solve the problem, but I am struggling to understand why placing the system over the pulley makes this difference.
Please see the attached image.[]()[]()

![In the image, I have shown how to visualise the system when two masses of weight 15 N and 20 N are hung with the same massless string, and found out that the tension in the string is not 15 N but 17 N.](https://physics.codidact.com/uploads/a95c4pcbssflaqvdkvbjohdvlu9r)I'm working on a problem involving a pulley system, and I’m confused about how the tension changes when forces are replaced by masses.
Initially, if I apply forces of 20 N and 15 N directly at the ends of a massless string (without any pulley or masses), the tension in the string is exactly 15 N, as expected.
However, when I introduce masses (which exert 20 N and 15 N due to gravity) and place them over a pulley, the tension comes out to be 17 N instead of 15 N. I understand how to solve this mathematically, but why does the tension increase to 17 N when masses are used, and why does the pulley play such a crucial role?
I tried working through the mechanics, but I can't quite grasp the intuition behind the change in tension. I've attached an image of my attempt to solve the problem, but I am struggling to understand why placing the system over the pulley makes this difference.
Please see the attached image.[]()[]()

#1: Initial revision by

Thor ‭ · 2024-10-05T15:36:00Z (about 2 months ago)

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Why does tension change from 15 N to 17 N when forces are replaced by weights?

I'm working on a problem involving a pulley system, and I’m confused about how the tension changes when forces are replaced by masses.

Initially, if I apply forces of 20 N and 15 N directly at the ends of a massless string (without any pulley or masses), the tension in the string is exactly 15 N, as expected.

However, when I introduce masses (which exert 20 N and 15 N due to gravity) and place them over a pulley, the tension comes out to be 17 N instead of 15 N. I understand how to solve this mathematically, but why does the tension increase to 17 N when masses are used, and why does the pulley play such a crucial role?


I tried working through the mechanics, but I can't quite grasp the intuition behind the change in tension. I've attached an image of my attempt to solve the problem, but I am struggling to understand why placing the system over the pulley makes this difference.

Please see the attached image.[]()[]()

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