Higher-education textbooks1–4 state that forces as physical entities are independent of the frame of reference, whenever it is inertial. It can be shown that the acceleration and Newton’s laws are invariant by Galilean transformations, but this fact is hardly addressed beyond the formalism. We designed an instructional experiment, based on images from videos of gliders sliding on an air track, to contextualize the conditions of application of Newton’s laws, the meaning of Galilean transformations, and the rationale of the second law. The interpretation provided here focuses on possible cause–effect relationships and the assumption of time simultaneity, and is aimed at teachers of introductoryphysics courses.

The invariance of acceleration with the inertial frame of reference is approached in physics classes with deductions of expressions showing it; the algebraic simplicity of the “transformations” involved hides the profundity of the concepts and the importance of Galilean invariance in the architecture of Newtonian...

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