An object and a concave mirror of focal length f = 10 cm both move along the principal axis of the mirror with constant speeds . The object moves with speed `V_(0)=15 cm s^(-1)` towards the mirror with respect to a laboratory frame . The distance between the object and the mirror at a given moment is denoted by u . When u = 30 cm , the speed of the mirror `V_(m)` is such that the image is instaneously at rest with respect to the laboratory frame , and the object forms a real image .The magnitude of `V_(m)` is _______ cm `s^(-1)` .