Note that the body and its adjacent bodies mutually exert forces on each other (according to Newton’s third law).įig. Figure 4.1 shows the FBD of a body in contact with other adjacent bodies. An external force (or a distribution of force) originating from an implied adjacent body and acting on a free body can be referred to as a load. the effects of wind (surroundings) on a building (body) are wind forces to be considered on the walls of the building. For example, field forces, like the weight of the object. Sometimes, a body adjacent to another body is implicitly considered and its presence is expressed by its effects on the free body. These forces, called contact forces, are due to the contact interactions between the bodies. To construct the FBD, the body is isolated from its contact points (regions) with the adjacent bodies, and the contact forces and couple moments are shown on the body. A body is surrounded and physically attached or in contact with a group of adjacent bodies. Based on the way a body is chosen, three scenarios can be considered for constructing the FBD of a body:ġ- A single body. These forces do not appear (or affect) a FBD as they are inside the body itself and not externally exerted by the surroundings. We will learn more about the resultants of internal forces in chapter 7. The internal forces are forces between the particles of a body. In addition to the external forces on a (free) body, there are internal forces within the (free) body. A graphical demonstration of a free body subjected to external forces is referred to as a free-body diagram (FBD). For example, the effects of gravitational, electrical or magnetic fields exert external forces at internal points of a body. The external forces can act on the outline (outer surface) or at internal points of a body. The forces external to a free body exerted by its surroundings are referred to as external forces. A part of a cable under tension is a free body and the rest of the cable is in its surroundings.A building (isolated from the earth) is a free body and the earth is in its surroundings.If we consider the earth as a free body (a particle in fact) then the rest of the things in the universe are its surroundings.The scope and shape of a free body depends on our intention about the part that we want to study. The law states that if a body exerts a force (and/or a couple moment) on a second body (the surroundings), then the second body exerts a force (and/or a couple moment) with the same magnitude but opposite direction on the first body. The force/couple moment interactions between a free body and its surroundings obey Newton’s third law of motion. A free body and its surroundings interact by exerting forces and/or couple moments on each other. In engineering mechanics, an isolated body is referred to as a free body. To analyze the forces on a body and their effects, one technique is to imaginarily isolate the body from its surroundings, i.e. Open Educational Resources Engineering Mechanics: Statics:Ī body is a physical object it can be anything from a building, car, bolt, or even a piece of another body for example, a piece of a cable or any other structures. Relationships between Load, Shear, and Moments.Shear and moment equations and their diagrams.Conditions for two dimensional rigid-body equilibrium.Equilibrium of Particles and Rigid Bodies.Simplification of force and couple systems.Vector operations using Cartesian vector notation.Vector operations using the parallelogram rule and trigonometry.
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