Under this heading John Denker is not resenting a summary of recent
contributions on this topic to the list but a summary of his own
rather eccentric interpretation of Newton's laws of motion. In so
doing he seems to be seeking to revive this dying thread.
I think that most readers of the thread would be subscribers to half
of what he describes as a Big-End version of the laws: the version
that states that a net unbalanced force on an object causes a change
in the motion of that object. In other words we subscribe to the
first of Newton's laws.
John seems to be caught up with a fixed idea that F = ma is all there
is to Newton's laws. I see this mathematical statement as a recipe
for calculating the acceleration of an object of mass m acted on by a
net force F: one that is more properly written a = F/m. It is the
second law which follows on from the first law: the first law has said
why changes in motion occur; the second tells us how to calculate
these changes. As I wrote, a recipe.
It is a recipe that can, if circumstances warrant it, be used to
calculate the value of a net force from the knowledge of the mass of
the object and a measure of the observed acceleration. Such a
calculation does not in any way imply that the acceleration was
responsible for or caused the force. Similarly the recipe can, if
circumstances warrant it, be used to calculated the mass of an object
if the net applied force is measured or known and the resulting
accelerating measured. Again this calculation does not in any way
imply that the acceleration and the force were responsible for value
of the mass.
John argues for a mathematical symmetry of this recipe, giving the
applied forces and the resulting acceleration equal billing in his
model of why changes occur in the world.
Such an approach ignores the first law:"Every body continues in its
state of rest, or of uniform motion in a right line, unless it is
compelled to change that state by forces impressed upon it."
John also seems to give a very narrow interpretation of words of David
Hume that effectively say that effect cannot precede cause. On this
basis John believes that the effect cannot be simultaneous with the
cause. Again, I doubt if most persons on the list would agree with
him. I certainly do not.