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Re: CAUSATION IN PHYSICS



I'd like to comment on one of the points John Denker has made during
this thread:


"Suppose we have a wheel spinning on an axle at some rate $omega$.
Everything >would be in balance, except that we have attached to the
wheel a mass $m$ at a distance $r$ from the center. Calculate the
force.

It would be natural to calculate $F$ in terms of $ma$ in this case.
It would be bizarre to calculate $a$ in terms of $F/m$ in this case."


Of course we would use ma to calculate F. We can observe the
acceleration of the attached object (BTW why call it a mass and not an
object; mass is the name of one of the object's quantities, not the
object itself; but that is a different story).

We can then use that acceleration, the known mass of the object, and
the relation F=ma to calculate the value of the contact force exerted
by the wheel on the object.

This does NOT mean that the acceleration is causing the force. F=ma
is being used here merely to calculate what force is necessary to
produce an observed acceleration. What is causing this observed change
of motion (Newton's first law) is a contact force between the wheel
and the object that has arisen in the attachment process that John
refers to. If the contact force is not strong enough the object will
not travel with the wheel.

Sometimes, in many textbooks, this contact force is called a
centripetal force; this nomenclature leads to much confusion among
students who believe that it is an extra force, but that again is
another story.

The moral behind this example is that the mathematical relation F=ma
does not replace Newton's laws of motion. Words, not mathematical
symbols, are need to give a full picture of this basic physical model.

Brian Mc