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For example, if I am to find the tension in a rope between two objects, do I use the mass of the two objects or of the whole system?

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- Thread starter z-component
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- #1

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For example, if I am to find the tension in a rope between two objects, do I use the mass of the two objects or of the whole system?

- #2

Doc Al

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It depends on what you are trying to find. If you are trying to find a property of the entire system (say the acceleration of the center of mass), then you'd use the mass of the entire system.z-component said:How do I know if I am supposed to add two masses or just use the one object's mass depending on what's asked?

In this case, the only way to find the tension is by analyzing the masses separately. (If you treat them as a single system, the tension will be an unknown internal force.)For example, if I am to find the tension in a rope between two objects, do I use the mass of the two objects or of the whole system?

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- #4

Doc Al

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Realize that if you take both objects together as a single system, you won't be able to learn anything about the forces between them, since they will be internal forces (and, per Newton's 3rd law, they will cancel out). This will all become obvious after you've solved a few hundred problems.

But, as I said earlier, often it will prove useful to analyze the problem both ways: As a composite system

- #5

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Thank you, that helps. I am looking forward to the 100th time I solve a mechanics problem... ;)

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