Free Body Diagram:

Let’s introduce the first basic concept in statics, free body diagram. See figure below:

as can be seen here, a glass rests on a table. For the purpose of statics, this can equally be represented as on right part of the figure. The weight of the glass exerts a force on the table, plus, there is the self weight of the table, which can be shown as a single force acting on the center of mass of the table. And the floor exerts reaction forces upwards to the legs of the table. So this table is in equilibrium right now, it does not move. The right side of the figure is called a “Free Body Diagram” of the table, which shows the table itself plus all the external forces acting on it. The reaction forces from the floor here are sufficient to counter the weight of the glass plus the weight of the table and therefore the table does not move, and is in static condition, and therefore the name statics, for this subject. This is the starting point of how we solve forces acting on bodies at the most basic level in a statics class.

Also, an important thing to learn here is that in reality the glass exerts a uniform force to the table over its base area to the table, and the table’s top part and legs also has uniformly acting weight distributed over the table’s geometry, but for the purpose of statics, we can simplify all these uniform forces to single point forces as shown here. Think about the figure on the right, versus a figure where we try to represent all the uniform forces along the base of glass, and the body of table. Which one is simpler? And this still gives us the correct solutions for our purposes.   

One more important thing to learn here is that we assume all bodies are rigid bodies in statics. In other words, they are infinitely strong and they do not break. For example, here we assume that our table is rigid, and will not deform or break. In nature such material does not exist, but for our purposes, it is often times very useful in structural engineering, and within tolerances, to assume that members are rigid, which greatly simplifies the solution, as we see here. But this can only be done for simple problems, to visualize things. For a complete structural analysis, you cannot assume everything is rigid, except for relatively very strong members with respect to other members, in order to simplify calculations within tolerances. 

In the next post, we will introduce the term “Forces”

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