I am working on a model where the top-half of an assembly is being accelerated upwards by a cylinder-piston assembly. So, I need to apply acceleration ONLY to the top-half of the assembly that's being lifted, and not the rest of the body. In classical mechanics, solving for F = ma for the top-half of the assembly free body diagram would give me the force from the piston rod to top-half of the body as: F = m(g+a), were "a" is the acceleration with which the body is being pushed upwards, and "g" being the acceleration due to gravity.
If you need some details about the cylinder-piston assembly modeled in my ANSYS model: they are modeled simply as two beams with a body-body general joint with translational and rotational dof free about the axis of the cylinder-piston.
Another case: if I have, say an iron block, suspended from a non-metallic rod, and the iron block experiences a body force due to some magnetic field pulling it down. I need to solve for the reaction force at the connection between the top of the non-metal rod some external object it's connected to. So, the acceleration is to be applied ONLY to the iron block and not the non-metallic rod.