F is a forcing function, not the resultant force. Itβs often arranged this way with all the derivatives on one side (as opposed to having the resultant force, ma, on one side of the equality by itself) so that it matches the general form of a non-homogeneous second-order linear differential equation.
(At least I assume this is what the original commenter meant!).
Ultimately it all comes down to choosing the most convenient basis functions for the questions you're answering.