This claim is a fundamental physical principle derived from the linearity of equations of motion in classical mechanics (Newton's second law for linear systems), making it a matter of common knowledge that does not require specific paper citations.
The claim states that acceleration is a linear function of external forces due to the superposition principle. In standard linear systems and Newtonian mechanics, the equation of motion is linear ($F = ma$, hence $a = F/m$), meaning acceleration scales linearly with force and satisfies superposition. This is a foundational textbook definition and common knowledge in physics, rendering citations unnecessary to establish its truth.