Radiation heat transfer, on the other hand, is a perfect example of a body force.
Rather they are corrections to Newton's second law when it is formulated in an accelerating reference frame.
(Gravity can also be considered a fictitious force in the context of General Relativity.)
The body force density is defined so that the volume integral (throughout a volume of interest) of it gives the total force acting throughout the body; where dV is an infinitesimal volume element, and f is the external body force density field acting on the system.
For a non-rigid object, Newton's second law applied to a small volume element is where ρ(r) is the mass density of the substance, ƒ the force density, and a(r) is acceleration, all at point r. In the case of a body in the gravitational field on a planet surface, a(r) is nearly constant (g) and uniform.