Free Body Diagrams

"Free body diagrams" are diagrams that identify all forces on an object. Done correctly, the free body diagram sets up the solution to a problem of forces. The following is one set of guidelines to setting up a successful free body diagram:

1. Draw a small circle, or dot to represent the center of mass of the object on which the forces are acting (a picture of the object is NOT necessary).

2. Label each force carefully. The arrow that represents each force should start at the center of mass and point out of the object.

3. At this point you have completed the free body diagram.

***Note: free body diagrams DO NOT contain a vector representing the net force; also, free body diagrams do not have components drawn on them...the components are part of the solution to the problem; they are a tool...the diagram is the starting point for the problem.

4. Decide on the plane in which the object might accelerate (on a ramp, it would be the plane parallel to the ramp). To help, draw dotted lines to show this preferred coordinate axis system with its origin at the center of mass of the object.

5. You should notice that some of the forces line up nicely with one or the other of the axes of the coordinate system; those forces that DON'T line up with an axis should be resolved into 2 perpendicular (orthogonal) components that DO line up with the two axes.

***Note: each force should be drawn as close to its correct relative length when compared to the other forces. This will help to determine the direction of acceleration of the object.

6. You are now ready to apply Newton's Second Law and the definition of "Net Force" to the problem to solve for a missing force or the acceleration.

F_net = ma
F_net = F₁ + F₂ + F₃ + ... etc.