Can we write equations for strings in pulley systems?

[Kartik.]

Ofcourse we can write the equations for blocks on a single fixed pulley taking the tension upwards and weight as downwards and then equating according to the acceleration. Can we write equations for the string (massless or with mass)?

 

[mfb]

Equations for what, where?
I would expect that the answer is "yes", but the question is quite unclear (at least to me).

 

[Pkruse]

If someone can rig up a system of pullies, then we can write the equations for that system. Why don't you suggest an example?

 

[PhanthomJay]

If the string has mass, its tension will be variable (not constant), but you can still apply Newton's laws to it. If it has no mass, it just serves to transmit forces between objects with mass, at a constant tension. There cannot be any net force acting on it, although it would still accelerate along with the object it supports.