I understand that if a string is holding up a hanging mass then the magnitude of the tension in the string is mass * gravity. The other end of the string is tied to an object on a flat surface (after being redirected by a frictionless pully). If the tension force is great enough to overcome static friction then the object, string, and mass will move. If I know all the relevant weights and coefficients of friction is there a way to calculate the magnitude of the tension in the string? It must be less than mass * gravity because the mass is being pulled down. It can't be the same as the frictional force slowing the object because the object is moving too.