EM 274: Do & Dont Notes for 2-D Free Body Diagram Sketches

  1. A FBD sketch should be an outline of a part or all of the system being studied.
  2. If you require only support reactions
    you can use the complete device for the FBD
    If you require forces on parts
    draw a separate FBD for each part that is not rigidly connected to other parts. e.g., at a pinned joint (hinge)
  3. Label all forces/couples, applied and support reactions or indicate values if known
  4. Label all body geometry (distances and angles), or indicate values if known
  5. Support mechanisms should not be part of the sketch, only the forces or moments they exert
  6. Show coordinate axis sytem
  7. this is the one you will use for your solution analysis
  8. centered to make solution process simplest, usually at a support point
  9. aligned to make solution process simplest; usually one of the axes passes through support points
  10. not necessarily the same axes used in system diagram
  11. Check for adequate constraints
  12. FB is prevented from moving (translating or rotating)
  13. reactions are not partially constrained (all support forces must not be parallel or concurrent)
  14. Check FBD is not over-constrained
    (i.e. statically determinate, no redundant constraints)
  15. can any constraint(s) be removed and FB still be adequately constrained?
  16. if the direction of a force is known at a support point show a single labelled force in its proper direction and a value for the angle
  17. if the magnitude of a force is known at a support point show its value on the vector and label the unknown angle
  18. for either of the two previous cases do not show cartesian components; this will make the body appear to be over-constrained
  19. At the join between two FBs, check that, if put back together, they will result in the original force system at the join