A vector quantity equal to the rate that velocity changes with time. A system of internal forces whose resultant is a force acting along the longitudinal axis of a structural member or assembly. This term is most commonly used to refer to internal forces in beams.
Gravity is a body force.
An inertial force is a body force. A brittle structure or material exhibits low ductility , meaning that it exhibits very little inelastic deformation before complete failure. Similar to the concept of center of gravity , except that it applies to a two dimensional shape rather than an object. For a given shape, the centroid location corresponds to the center of gravity for a thin flat plate of that shape, made from a homogeneous material. Any vector can be expressed as a collection of vectors whose sum is equal to the original vector.