# Forces44.pdf - Three rules that help to establish the...

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Three rules that help to establish the direction of frictionless contact forces are: (1) When one of the two contacting surfaces is flat , the force must act perpendicular to the flat surface; (2) When two solids contact along sharp edges, the contact force must be perpendicular to both edges. (3) When two curved surfaces contact, the reaction force acts along a line joining the centers of curvature of the two objects. Forces acting at rough (infinite friction) nonconformal contacts A rough nonconformal contact behaves somewhat like a pinned joint. There can be no relative motion of the contacting surfaces, therefore there must be three components of reaction force acting on both contacting solids. Unlike a pin joint, however, the contact can only sustain a repulsive normal force. This means that the components of force shown in the picture must satisfy 0 A N . If the normal force is zero (eg when the two surfaces are about to
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