Observations show that interstellar clouds can have almost any shape and, if they are rotating at all, their rotation is not perceptible. however, as shown in the video, the nebular theory predicts that a cloud will rotate rapidly once it shrinks to a small size. what physical law explains why a cloud will rotate rapidly as it collapses?

The law of conservation of angular momentum holds because the cloud is collapsing only due to gravity.Because angular momentum is proportional to velocity times radius, keeping this product constant requires that the velocity of rotation increase when the cloud radius decreases. Hope it helps. 

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.