Engineering Mechanics Force System

FORCE SYSTEMS

A force has been defined as an action of one body on another. But in dynamics, a force is an action which tends to cause acceleration of a body. A force is a vector quantity, because its effect depends on the direction as well as on the magnitude of the action. Thus, forces may be combined according to the parallelogram law of vector addition. The action of the cable tension on the bracket in Fig. 2/1a is represented in the side view, Fig. 2/1b, by the force vector P of magnitude P. The effect of this action on the bracket depends on P, the angle q, and the location of the point of application A. Changing any one of these three will alter the effect on the bracket, such as the force in one of the bolts which secure the bracket to the base, or the internal force and deformation in the material of the bracket at any point. Thus, the complete specification of the action of a force must include its magnitude, direction, and point of application, and therefore we must treat it as a fixed vector.

External and Internal Effects

We can separate the action of a force on a body into two effects, external and internal. For the bracket of Fig. 2/1 the effects of P external to the bracket are the reactive forces (not shown) exerted on the bracket by the foundation and bolts because of the action of P. Forces external to a body can be either applied forces or reactive forces. The effects of P internal to the bracket are the resulting internal forces and deformations distributed throughout the material of the bracket, and depends on the material properties of the body and is studied in strength of materials, elasticity, and plasticity.

Principle of Transmissibility

When dealing with the mechanics of a rigid body, we ignore deformations in the body and consider only the net external effects of external forces. In such cases, experience shows us that it is not necessary to restrict the action of an applied force to a given point. For example, the force P acting on the rigid plate in Fig. 2/2 may be applied at A or at B or at any other point on its line of action, and the net external effects of P on the plate will not change. The external effects are the force exerted on the plate by the bearing support at O and the force exerted on the plate by the roller support at C. This conclusion is summarized by the principle of transmissibility, which states that a force may be applied at any point on its given line of action without altering the resultant effects of the force external to the rigid body on which it acts. Thus, whenever we are interested in only the resultant external effects of a force, the force may be treated as a sliding vector, and we need to specify only the magnitude, direction, and line of action of the force, and not its point of application.

Force System

A group of two or more forces is known as force system. From engineering mechanics point of view, a system of forces can be classified as:

		System of Forces

Coplanar Forces                                            Non-Coplanar Forces

      (Lying on the same plane)                             (Do not lie on the same plane)

Coplanar forces may be

·        Collinear

·        Concurrent

·        Non-Concurrent

·        Parallel

·        Non-Parallel

Non-Coplanar forces may be

·         Concurrent

·         Non-Concurrent

·         Parallel

·         Non-Parallel

TYPES AND CHARACTERISTICS OF FORCE SYSTEM