Lets say we have a frictionless and massless pulley with a massless string over it. There are two masses attached to it *m1* and *m2* where **m2** >**m1**.

So m2 would an experience an acceleration down while m1 experiences an acceleration up and massless string would also move accordingly.

However, if the net force on a massless string is always zero, it shouldn't experience any acceleration and stay at rest. But the string does move. How does it move?