# Why is tension an external force?

velvetmist
When my professor sums external forces (to know if the impulse is constant or not) he always includes tension.
I will use as an example pic related. If my system includes the two particles and the thread, woudn't tension being a internal force?
I'm sorry if it's a silly question, but I also asked it to some friends and they didn't know the answer either. Thank you for your time.

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• Delta2

Homework Helper
Gold Member
Yes, when you look at the system of both particles and the thread, the tension force in the thread is an internal force. But what your professor is likely doing is looking at a free body diagram of one of the particles, not both. In a free body diagram, you isolate the particle in question from the rest of the system by making an imaginary cut through the thread (in this case)and around the particle. The tension on the 'cut' thread now becomes an external force on the particle. Anytime you cut an internal force in a free body diagram, it now becomes external in that diagram.

• Delta2
velvetmist
Yes, when you look at the system of both particles and the thread, the tension force in the thread is an internal force. But what your professor is likely doing is looking at a free body diagram of one of the particles, not both. In a free body diagram, you isolate the particle in question from the rest of the system by making an imaginary cut through the thread (in this case)and around the particle. The tension on the 'cut' thread now becomes an external force on the particle. Anytime you cut an internal force in a free body diagram, it now becomes external in that diagram.

Thank you for answering! I don't think so tho. Of course he did the FBD but the sum of the external forces is to check energy conservation, so i haveto take into account the whole system.
The first pic is the FBD. N: normal force; P: mg; T: tension force.
In the secon one, the sum of the external forces. Fv would be something like link force, but i supose that in english you just call them both tension.

Now that I think about it, Fv could be the force between the table and the thread. In that case I'm such an idiot lol

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If my system includes the two particles and the thread, woudn't tension being a internal force?
Yes, if the system includes the two particles and the thread then the tension is an internal force. However, don’t forget that the system can be chosen or defined as desired in order to answer the question, or even multiple systems can be considered. For instance, if a system is defined to be only one particle then the tension force is an external force.

• Chestermiller