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I have a question related to how the reading of the balance is equal to the tension of the string.

In my opinion, the reading of the balance should be twice as much as the tension. This is the diagram I drew:

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Here isn't it very obvious that there are $2$ forces acting on the spring and pulling it in opposite directions, each with a magnitude $10N$? Therefore in my opinion, the reading $= 20N$.

But of course, it isn't so, the reading is $=10N$. I want to know what is wrong in my reasoning.

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    $\begingroup$ Funnily enough, that this question is a duplicate of the question I linked to answers the questions :) You can play this game ad-infinitum and it should be a sign that you are doing something fishy. The linked question suggests that the tension in the rope should be twice the force, you further suggest that the reading should be twice the tension that comes from the same confusion. You can keep doing this indefinitely because you can keep dividing the remaining string. ;) $\endgroup$
    – user87745
    Commented Mar 18, 2021 at 23:53
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    $\begingroup$ You have a hidden assumption about "the way that it should be" that isn't the way that it actually is. After teaching high school physics for more than a decade, I note that such "common sense" concepts are VERY difficult for students to discard. You must find a way to rethink such a problem, such that you can get rid of how things should be, and internalize the way that things really are. And note - your diagrams are related to Newton's 3rd law. $\endgroup$ Commented Mar 19, 2021 at 1:13

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Imagine the balance is attached to the ceiling by a string and a $10$ N mass is hung from the other end. You would say the reading on the balance is $10$ N. Neither the balance nor the mass is not accelerating.

Consider a free-body diagram of the balance. The string attached to the ceiling is pulling up with some magnitude force, $F_1$. The string attached to the $10$ N mass is pulling down. You should be able to calculate what $F_1$ is. That should tell you about your situation and how spring balances work.

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  • $\begingroup$ do you want to say that both the ends of string in this case are not connected to spring inside balance one is connected to hook and other one is connected to the spring inside balance $\endgroup$ Commented Jul 1, 2021 at 13:31
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This problem depends quite a bit on knowing how the balance is calibrated. Take the force diagram you've drawn here and compare it to the "normal" case in which the balance is attached to the ceiling and has a weight hanging on the bottom, which is a situation in which we know that the balance is supposed to read the weight of the object.

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