Free-body diagrams


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Free-body diagrams provide a standard way of describing mechanics problems in a way that is understood across languages. It omits details about what the objects are, and focuses entirely on the external forces acting on the object.

Here are rules to follow when constructing an FBD (free-body diagram):

  1. The FBD is separate from a sketch of the model you are working on.
  2. Represent the object with a dot.
  3. Show only external forces. Every force is depicted as an arrow starting at the dot, and pointing away. If you are thinking of the force as a push on the object, depict it as a pull from the other side of the dot.
  4. If you can, make the length of each force arrow proportional to the magnitude of the force. If you're not sure, do your best.
  5. Label each force with an , adding a descriptive subscript. For example, gravity can be , tension in a rope can be , et cetera.
  6. Choose reference direction(s), and indicate it on your FBD. If all vectors lie on a line, you will need only one dimension. For vectors that require a plane, indicate two directions, and make sure they are orthogonal.

Watch the videos below, and complete these guided notes.

  1. Free-body Diagram Rules 1-6
  2. Free-body Diagrams #7
  3. Free-body Diagrams #8
  4. Free-body Diagrams #9
  5. Free-body Diagrams #10
  6. Free-body Diagrams #11
  7. Free-body Diagrams #12

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