2. Augmented physical props: balance marble game. Figure 1 (see top) depicted our MR marble maze, which was in fact inspired by that of Ohan and Feiner and complemented with EMS-based force feedback for added realism. As previously explained, our marble game renders the shift in gravity caused by the moving marble by actuating the user’s arms towards the heavier side. Note that even if the marble perfectly balanced on the center of the tray we render a constant pull down of the user’s arms (triceps) to represent the marble’s weight under gravity. Furthermore, we render a number of extra haptic effects: (1) when the ball collides with any of the maze walls, we render a short bump in the user’s triceps, depending on the ball’s velocity vector; (2) when the marble falls out of the tray in one of the openings in the obstacle walls, the user feels the relief in the weight; and, (3) when the game starts, the marble falls from the sky onto the tray and we render this “extra weight” by quickly pulling down the user’s arms, as depicted in the Figure below.