The important things to know for Free Body Diagrams are:

  • The misconception that a contact force can apply a force without touching the system of interest.
  • The axis of the system is not set in stone. You must choose it for every free body diagram you draw.
  • The misconception that if there are two systems that I draw a free body diagram for they must have the same axis. You choose the axis that is most advantageous for each system, typically aligning the axis with the acceleration.