From where does a massless inextensible string gets energy to do work?

 We all know that to do work, energy is needed. But then, how can a massless string do work on an object, as if the string is massless and inextensible both gravitational and elastic potential energy is absent. Let's consider a simple Atwood machine in which two blocks are hung on either side of a pulley with the help of a massless string.

Now, if the masses of the blocks are unequal then the system would accelerate now if we consider the system to accelerate anticlockwise then, in that case, the work done by tension on the 2kg block would be positive, but if we have a closer look on the situation then we can observe that the work done by tension on the 5kg block is negative, so it is gaining energy at one side and using that energy to do work on the other side, or in other words, the net work done by the string equals to 0, so the string does not need any energy.

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                        -Arvin Gupta