Static

Engineering mechanics "Static"

lecture 1

Force System

Before dealing with a group or system of forces, it is necessary to examine the properties of a single force in some detail, A force has been define as an action of one body on another. In dynamics we will see that a force is defined as 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, the forces may be combined according to the parallelogram taw of vector addition. The action of the cable tension on the bracket in Fig.1a is represented in the side view,.Fig.2b, by the force vector P of magnitude P. The effect of this action on the bracket depends on P, the angle θ, and the location of the point of application A. changing any one of these three specifications will alter the effect on the bracket, such as the forces in one of the bolts which secure the bracket to the base, or the internal the complete specification of the action of a force must include its magnitude, direction, and point application, and therefore we must treat it as a fixed vector.

Figure 1

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 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. The rotation between internal forces and internal deformations depends on the material properties of the body and is studied in strength of materials, elasticity, and plasticity.

1

Principle of transmissibility

When dealing with the mechanics of a rigid body, we ignore deformations in the body and concern...