Free body diagram

In physics and engineering, a free body diagram (force diagram, or FBD) is a graphical illustration used to visualize the applied forces, movements, and resulting reactions on a body in a given condition. They depict a body or connected bodies with all of the applied forces and moments, as well as reactions, that act on that/those body(ies). The body may consist of multiple internal members, for example, a truss, or be a compact body such as a beam. A series of free bodies and other diagrams may be necessary to solve complex problems.

Free body diagrams are used to visualize the forces and moments applied to a body and calculate the resulting reactions, in many types of mechanics problems. Most free body diagrams are used both to determine the loading of individual structural components as well as calculating internal forces within the structure in almost all engineering disciplines from Biomechanics to Structural. In the educational environment, learning to draw a free body diagram is an important step in understanding certain topics in physics, such as statics, dynamics and other forms of classical mechanics.

A free body diagram is not meant to be a scaled drawing. It is a diagram that is modified as the problem is solved. There is an art and flexibility to the process. The iconography of a free body diagram, not only how it is drawn but also how it is interpreted, depends upon how a body is modeled.

Free body diagrams consist of:

The number of forces and moments shown in a free body diagram depends on the specific problem and the assumptions made; common assumptions are neglecting air resistance, friction and assuming rigid bodies. In statics all forces and moments must balance to zero; the physical interpretation of this is that if the forces and moments do not sum to zero the body is in motion and the principles of statics do not apply. In dynamics the resultant forces and moments can be non-zero.

Free body diagrams may not represent an entire physical body. Using what is known as a "cut" only portions of a body are selected for modeling. This technique exposes internal forces, making them external, therefore allowing analysis. This technique is often used several times, iteratively to peel back forces acting on a physical body. For example, a gymnast performing the iron cross: analyzing the ropes and the person lets you know the total force (body weight, neglecting rope weight, breezes, buoyancy, electrostatics, relativity, rotation of the earth, etc..). Then cut the person out and only show one rope. You get force direction. Then only look at the person, now you can get hand forces. Now only look at the arm to get the shoulder forces and moments, and on and on until the component you intend to analyze is exposed.

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