Question
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 instantaneously at rest with respect to the laboratory frame, and the object forms a real image. The magnitude of V m is _______ cm   s - 1 .