Cosmic-string equations of motion are derived with the force of friction due to string-particle scattering taken into account. For strings in a Robertson-Walker space, the additional terms in the equations of motion have the same functional form as the terms due to the expansion of the Universe. As a result, a computer simulation of strings with friction would require only a trivial modification of the existing programs.

• Received 20 July 1990

DOI:https://doi.org/10.1103/PhysRevD.43.1060