"These equations require that a rigid body will be in equilibrium provided the sum of all the external forces acting on the body is equal to zero and the sum of the moments of the external forces about a point is equal to zero... Many types of engineering problems involve symmetric loadings and can be solved by projecting all the forces acting on a body onto a single plane. Hence in the next section, the equilibrium of a body subjected to a coplanar or two-dimensional force system will be considered." p195

CENTER OF GRAVITY

EXTERNAL
COMING FROM OUTSIDE THE SYSTEM UNDER STUDY. IN MANY PROBLEMS, IT IS IMPORTANT TO KEEP TRACK OF RESOURCES OR FACTORS INSIDE AND OUTSIDE A SYSTEM BOUNDARY.

EXTERNAL FORCES

FORCE
A PUSH OR A PULL

FORCE

GRAVITY
THE ATTRACTIVE FORCE BETWEEN ANY TWO MASSES

INTERNAL
WITHIN A SYSTEM

INTERNAL FORCES
FORCES THAT ARE EXERTED BETWEEN PARTS OF THE SYSTEM

REACTION
A FORCE WHICH IS A RESPONSE OF A STRUCTURE OR ASSEMBLY TO AN APPLIED LOAD

SUPPORT REACTION

SUPPORT REACTIONS