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On 2019/Feb/07, at 19:25, Jeffrey Schnick <JSchnick@Anselm.Edu> wrote:
I've been thinking about this example (Newton's 2nd Law) and other continuity equations. I see Newton's second law as a continuity equation which I see as being synonymous with a conservation law. For some system, say a brick, the force on the brick is the rate at which momentum is flowing into the brick and the rate of change of momentum of the brick is what we often represent as the mass times the acceleration of the brick. The total momentum of the brick is changing right there at the boundary where the incoming momentum is crossing the boundary. Any change in the total amount of momentum inside the boundary (between the closed surface of the brick and the rest of the universe) is occurring at the exact same instant as the momentum crosses the boundary. I can't see any cause and effect when the two things occur simultaneously. Is the momentum of the brick changing because momentum is flowing across the boundary or do we judge there to be momentum flowing across the boundary because it is decreasing at the boundary on one side of the boundary and increasing at the boundary at the other side of the boundary. Epstein, in his book, Relativity Visualized, refers to the statement that force causes acceleration as (something like) the great myth of the physics community. I confess to using the cause and effect language but I don't really think force causes acceleration.