Comments on Why does tension change from 15 N to 17 N when forces are replaced by weights?
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Why does tension change from 15 N to 17 N when forces are replaced by weights?
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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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.
No, it's not. You've got a problem without a solution, like dividing by 0. You can't apply 20 N to pull a string and then say the tension is 15 N. It is 20 N by definition of applying 20 N. Of course on the other end you have the same thing, resulting in 15 N. That's an impossible situation for a massless object. It simply can't be.
In the real world, there would be a net force of 5 N in one direction. That divided by the mass of the object would tell you its acceleration. In your case you have an impossible situation since the mass is 0.
why does the tension increase to 17 N when masses are used
Because of motion. The 1.5 kg mass is being accelerated, so more than just its weight of 15 N must be applied to it. This is of course assuming you are on earth where 1 kg weighs about 10 N.
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