An object and a concave mirror of focal length f=10cm 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 instantaneously at rest with respect to the laboratory frame, and the object forms a real image. The magnitude of V m is________cms -1