1 Dr Joseph Shapira Slipping the map under the cap 1.4.2014 Slipping the map under the cap Dr Joseph Shapira jshapira@netvision.net.il An abrupt pull of the map from the table leaves the table set (almost) intact. This is a familiar experience. An impressive demonstration of slipping the map under 24 table-sets over a 6 m long table, leaving the sets undisturbed (http://www.youtube.com/watch?v=vfnt8Sdj7cs#t=34 ) rose a vivid discussion in the physics-teachers community which deserves the following analysis. Initial state : a cap ( marked A in Figure 1) is placed on the map covering a table, a distance d from its end. Its mass is m A and its coefficient of static friction is µ AS . The map is designated B, its mass is m B and its coefficient of static friction with the table is µ BS . A horizontal force F is applied to the map (pulling the map). Figure 1: Initial state. Force diagram. The following evolves in sequential phases. For sake of simplicity the cap is considered stable (assuming it is a point mass). Phase 1 : As the pull starts, the force F increases. The map does not move until F reaches the maximal static friction force between the map and the table. ≤ (  +   )  (1). No horizontal force is applied to the cap, as the map does not move. Phase 2 : F surpasses the maximal static friction force between the map and the table: F m B g m A g f B = (m A +m B )gµ Bs A B שלחן d D N