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Re: Cause and Effect



Brian Whatcott has quoted Newton (in translation). I take it as evident
that Newton stated a causal relation. That is evident to me in his use
of the word "impressed" (I'm sorry, my Latin is gone). It is also evident
to me that forces can cause accelerations, and that forces can do other
things as well, like compressing springs. My students seem to have no
difficulty grasping this causal connection either. Why is there a problem?

When I said forces *can* cause accelerations I chose my word carefully.
It is not necessary that a force cause an accelleration. Only *unbalanced*
force causes acceleration. Let me set up a demonstration of a force which
can either cause an acceleration or not. Imagine you are blindfolded. You
punch a padded plate to compress a horizontal spring, the distal end of
which is firmly attached to a wall. Your punch is sufficiently brief that
it is over (the force has returned to zero) before the impulse reaches the
distal end of the spring.

The wall is mounted on wheels which, in turn, roll on horizontal rails in
the direction of your punch.

Case I: The wall, spring and pad acquire a momentum which is equal to the
impulse you imparted with your punch. The force was unbalanced by constraint.

Case II: the wheels are equipped with brakes. The brakes are set. The wall
does not move. The force (actually the impulse) was balanced.

You, the blindfolded causal agent, find any difference in the blow you have
struck to be imperceptible*. Spectators will affirm that you were the agent
which precipitated either result.

You've struck a blow for reason over neoscholasticism.

Leigh

* just so.