First, in your button (this assumes that your arrow has the instance name 'arrow'):

Code:
on (release) {
    if (!_root.arrow.spinning) {
        _root.arrow.spinning = 1;
        _root.arrow.speed = (math.random()*20)+30;
    }
}
Now in the Object actions of your 'arrow' movie clip:

Code:
onClipEvent (load) {
    friction = 1.1;
    rot = 0;
}
onClipEvent (enterFrame) {
    if (spinning) {
        rot += speed;
        this._rotation = rot;
        speed /= friction;
        if (speed <= .1 && spinning) {
            spinning = 0;
// *** Your actions for when the arrow stops spinning go here ***
        }
    }
}
You can fiddle with the 'speed' and 'friction' values to change how fast it starts rotating and how quickly it slows down.

[Edited by Stickman on 07-05-2001 at 09:16 AM]