If we could make a greater change in the acceleration, we could observe the twist in the response surface due to the interaction term. Possible model simplification. Interaction coefficients are correlated even if the design is orthogonal. Forward and backward stepwise regression sometimes terminates with different models. Adding terms or removing terms changes the values of coefficients in the model.
All coefficients are uncorrelated if the design is orthogonal.
Forward and backward stepwise regression always terminates with the same model. Adding terms or removing terms does not affect the values of coefficients in the model. The model can have more terms than absolutely necessary. In general, coded units are preferable. It is rare that a physical simplification like the one in this article actually happens.