An object and a concave mirror of focal length \( f = 10\,\text{cm} \) both move along the principal axis of the mirror with constant speeds. The object moves with speed \( V_0 = 15\,\text{cm}\,\text{s}^{-1} \) 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 \( u = 30\,\text{cm} \), 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 ____ \( \text{cm}\,\text{s}^{-1} \).
