Embodied experiences of the equivalence principle and the temptation of inertial forces

Journal article, 2026

According to the equivalence principle, acceleration cannot be distinguished from a gravitational field in the opposite direction. Although conceptually different, using inertial forces in a consistent way gives mathematically equivalent descriptions of the interactions required for acceleration. However, we find that while most new students have learned to avoid ‘centrifugal forces’, many of them are still tempted to draw or choose force diagrams including an outward force in some cases of circular motion, but referring to a ‘centripetal force’, and many others prefer a diagram with a centripetal force as a real force. Invoking the weak equivalence principle, which applies to objects with mass, can be a way to connect students’ bodily understanding with formal mechanics. In this way embodied experiences of acceleration during everyday life, sports and play can be used as resources for conceptual understanding. Including kinetic diagrams in the construction of free-body diagrams is a reminder that force equilibrium does not apply for accelerating bodies. The weak equivalence principle is a consequence of classical mechanics. It inspired Einstein to develop the general theory relativity.