Purpose of the Free Body Diagram
The purpose of the free body diagram is to deconstruct a given problem by using only the necessary information. Students can use this diagram as a reference for setting up the calculations to find unknown variables, for example, force directions, moments, or force magnitudes. Free-Body Diagram allows students to clearly visualize a particular problem in its entirety or closely analyze a particular portion of a more complex problem. So basically, FBD is a very useful aid to visualize and solve engineering problems. Note that, for solving a complex problem, a series of free body diagrams may be required.
Drawing a Free-Body Diagram
In a Free-Body Diagram, the object is represented by its expression, usually a line, box, or a dot. The force vectors that act upon the object are represented by a straight arrow while moments are represented by a curved arrow around their respective axis as shown in the image below where a force is acting at B and a moment acts around A. The force vector indicates the magnitude and direction of each of the force that is acting upon the object. The direction is normally indicated by degrees from the vertical or the horizontal axis while the magnitude is indicated by the units of force. In the case of an unknown direction of a force or magnitude, the unknown value must be labeled.
In addition, it is common to indicate the various types of forces with letters and distinguish between common ones by using the subscripts. In the example problem shown in Fig. 1, weight and tension are represented by W and T, and the normal force and force of friction are represented by Ffrict and Fnorm. There are no hard rules about how forces are labeled as long as their meaning is clear. Free-Body Diagrams should have a labeled coordinate system and must include all the given dimensions, such as length and angles. Generally, a xy-coordinate system is used; however, when dealing with a problem, that is, in three-dimensional space, a xyz-co-ordinate system is required. Coordinate systems can be placed according to the student’s discretion in order to simplify the solving process, so long as the students are following the right-hand rule.
