I have a generic question in abaqus explicit element formulation (VUEL). We have to define amass (consistent/lumped mass matrix), dTimeStable (stable time increment) and RHS (internal force vector F_int). Why is it that we dont need to define F_ext? I know the 2nd law is Ma=F_ext-F_int or
a=M-1*(F_ext-F_int) where F_ext is the surface traction and body force integrated over the element domain.
My question is why we dont need to define the F_ext ? How does abaqus know/integrate the assigned surface tractions?