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Skill 2 of 11

Force diagrams

When drawing a force diagram like the following, what are we then not drawing? What is left out? All the forces that influence the book are there, but which forces are not there?

The forces that influence the hands are not there!

Okay, that was obvious. But it is important. A force diagram is a drawing of all forces on one particular object. Whether or not the hands are drawn, doesn’t really matter since their influences are drawn as forces. We could skip the hands entirely and only draw the book with the forces acting on it:

In fact, we don’t even need the book! We know that we are talking about the book, so we can just draw a “doodle” to represent the book, and then draw the forces.

This “doodle” then represents the object, the body, and the forces represent the influence of the surroundings. The body is free from disturbances on the drawing. A force diagram could be called a free-body diagram as well. This is in fact a very often used term.[1,2]

Sometimes it might be important to know where on the object, the forces act (that will be important in e.g. the Rotation discipline). Then we cannot just draw the object as a “doodle” but must draw it with its correct size and maybe shape. But other times it is not important. Then the size and shape will just be unnecessary information, and we’ll just leave it out, if we wish. We can call our “doodle” a model for the object that represents the object.

We should forever keep in mind that such a force diagram is for one object only.1 We could draw another force diagram for each hand, another one for the ground etc. They would look different.


References:

  1. Sears and Zemansky’s Univesity Physics with Modern Physics’ (book), Hugh D. Young & Roger A. Freedman, Pearson Education, 13th ed., 2012
  2. What are Free Body Diagrams?’ (web page), Chris H. Luebkeman & Donald Peting, Massachusetts Institute of Technology, MIT, 1997, web.mit.edu/4.441/1_lectures/1_lecture14/1_lecture14.html (accessed Aug. 27th, 2019)