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Support polygon : ウィキペディア英語版
Support polygon

For a rigid object in contact with a fixed environment and acted upon by gravity in the vertical direction, its support polygon is a horizontal region over which the center of mass must lie to achieve static stability. For example, for an object resting on a horizontal surface (e.g. a table), the support polygon is the convex hull of its "footprint" on the table.
The support polygon succinctly represents the conditions necessary for an object to be at equilibrium under gravity. That is, if the object's center of mass lies over the support polygon, then there exist a set of forces over the region of contact that exactly counteracts the forces of gravity. Note that this is a ''necessary'' condition for stability, but ''not a sufficient'' one.
==Derivation==
Let the object be in contact at a finite number of points C_1,\ldots,C_N. At each point C_k, let FC_k be the set of forces that can be applied on the object at that point. Here, FC_k is known as the ''friction cone'', and for the Coulomb model of friction, is actually a cone with apex at the origin, extending to infinity in the normal direction of the contact.
Let f_1,\ldots,f_N be the (unspecified) forces at the contact points. To balance the object in static equilibrium, the following Newton-Euler equations must be met on f_1,\ldots,f_N:
* \sum_^N f_k + G = 0
* \sum_^N f_k \times C_k + G \times CM = 0
* f_k \in FC_k for all k
where G is the force of gravity on the object, and CM is its center of mass. The first two equations are the Newton-Euler equations, and the third requires all forces to be valid. If there is no set of forces f_1,\ldots,f_N that meet all these conditions, the object will not be in equilibrium.
The second equation has no dependence on the vertical component of the center of mass, and thus if a solution exists for one CM, the same solution works for all CM+\alpha G. Therefore, the set of all CM that have solutions to the above conditions is a set that extends infinitely in the up and down directions. The support polygon is simply the projection of this set on the horizontal plane.
These results can easily be extended to different friction models and an infinite number of contact ponts (i.e. a region of contact).

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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