The pulley is massless and frictionless, string, too, is ideal. Why does the book say that the tension in the green string is $2T$ if the tensions in the two wings of the lower string is $T$ and $T$. Like if we see closely the strings only apply the normal force on the pulley how is it equal to two times tension $T$.
Also if the pulley would have been having mass and friction props. (string is mass less but with friction) would the tensions in the lower string be the same throughout? And what about the upper string? And what happens if everything is non ideal?