Force diagram

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The force diagram of a body is a diagram that depicts all the forces (as vectors) acting on the body. A force diagram is also termed a free body diagram.

Two-dimensional case

Consider the case that the directions of actions of all the forces are in the same plane, i.e., there is a plane such that all the forces on the body are in directions parallel to that plane. In this case, the force diagram is typically drawn with that plane as a cross section, so that all the forces can be depicted in a manner that is faithful to their actual direction of action.

Key aspects of the force diagram

  • The force diagram should include only the forces on a single body. In particular, a force diagram cannot contain both of two forces that form an action-reaction pair as per Newton's third law of motion.
  • The force diagram should depict correctly the direction of each force, both in terms of the line and the sense along that line (forward or backward). This is typically done by making the force vectors as arrows pointing along the direction of the force.
  • Each force should be clearly labeled.
  • Forces obtained by taking axis components of the forces in the diagram should either not be drawn or should be drawn in a manner that clearly distinguishes them from the original forces.

=Optional aspects of the force diagram

  • The line of action of the force should be chosen correctly. This is not important for force diagrams intended for Newton's laws, but is important for torque diagrams, which are force diagrams drawn to set up equations for rotational motion.
  • Preferably, the size of the force arrows can be drawn proportionate to the magnitudes of the forces. However, this is not possible if the force magnitudes have not been computed in advance.
  • Only the body whose force diagram is being considered should be drawn, other bodies, if depicted at all, should be depicted simply in a way that highlights their interaction with the body whose force diagram is being drawn.