Scale-invariant string evolution with friction

We study the effects of friction on the scaling evolution of string networks in condensed matter and cosmological contexts. We derive a generalized ''one-scale'' model with the string correlation length L and velocity upsilon as dynamical variables. In nonrelativistic systems, we obtain a well-known L proportional to t(1/2) law, showing that loop production is important. For electroweak cosmic strings, we show transient damped epoch scaling with L proportional to t(5/4) (Or, in the matter era, L proportional to t(3/2)). A low initial density implies an earlier period with L proportional to t(1/2). For GUT strings, the approach to linear scaling L proportional to t is faster than previously estimated.