# "Free Body Force" Definition in SolidWorks

Question asked by John Willett on Jan 1, 2016
Latest reply on Oct 8, 2016 by John Willett

What exactly is a "free body force" as reported by SW under Simulation/Result Tools/List Free Body Force?

SW KB entry S-035204 says in part:  "In SolidWorks Simulation, ...and free body forces are used in accordance with their respective conceptual definitions in basic physics...  Free body forces... show the equivalent forces that are acting upon a body or portion of a body, including contact forces, external loads, restraints, and connectors. If you look at the free body force for an entire component, you should see that all the forces sum to zero if the body does not have any rigid body motion."  [Emphasis mine.]

This "definition" is not completely correct since there appears to be no way to select "an entire component;" and in any case it leaves the question open, especially with regard to gravity, "Inertial Relief," and other body forces.  Body and surface forces would normally appear together in the kind of "Free Body Diagram" found in elementary physics, often all acting at the center of mass of the body to determine its linear acceleration.  (Obviously I'm ignoring torques here for simplicity.)  As far as I can find with Google (which of course knows all), there are no "conceptual definitions in basic physics" for the term, "free body forces."

In this forum the best I can find is the thread at Free Body vs Reaction Force , in which Mauricio Martinez-Saez provides an beautiful discussion of reaction forces but skirts the question of free body forces.

Can anyone further clarify the meaning of the term?  My guess is that the "free body forces" reported when a particular surface is selected include contact forces and torques (normal and friction) + all external loads + reaction forces and torques + the small fraction of body forces that is associated with the nodes actually on that surface.  This could be pretty misleading when body forces are present.  (Interestingly, free body forces can be measured across bonded contacts as long as they are created as separate contact sets.  Global bonding is apparently not distinguishable from a single part.)

SW's use of the related terms "contact force" and "reaction force" appears pretty straightforward (except with regard to torques, which are apparently not included even when present) in that the former applies to other bodies (though only through contact conditions other than bonded), the latter to restraints, and neither measures external loads nor (I think!) fractional body forces.

Do I have it about right, or am I still muddled? -- John Willett