STATICS OF RIGID BODIES BASIC INFORMATION AND TUTORIALS

If the forces acting on a rigid body do not produce any acceleration, they must neutralize each other, i.e., form a system of forces in equilibrium.

Equilibrium is said to be stable when the body with the forces acting upon it returns to its original position after being displaced a very small amount from that position; unstable when the body tends to move still farther from its original position than the very small displacement; and neutral when the forces retain their equilibrium when the body is in its new position.

External and Internal Forces
The forces by which the individual particles of a body act on each other are known as internal forces. All other forces are called external forces.

If a body is supported by other bodies while subject to the action of forces, deformations and forces will be produced at the points of support or contact and these internal forces will be distributed throughout the body until equilibrium exists and the body is said to be in a state of tension, compression, or shear.

The forces exerted by the body on the supports are known as reactions. They are equal in magnitude and opposite in direction to the forces with which the supports act on the body, known as supporting forces.

The supporting forces are external forces applied to the body. In considering a body at a definite section, it will be found that all the internal forces act in pairs, the two forces being equal and opposite. The external forces act singly.

General Law
When a body is at rest , the forces acting externally to it must form an equilibrium system. This law will hold for any part of the body, in which case the forces acting at any section of the body become external forces when the part on either side of the section is considered alone.

In the case of a rigid body, any two forces of the same magnitude, but acting in opposite directions in any straight line, may be added or removed without change in the action of the forces acting on the body, provided the strength of the body is not affected.