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Re: [Phys-l] F causes a



Bob LaMontagne wrote:

In physics, one of the conditions for causality is that there is not only an
order to the events, but that there is no frame of reference from which that
order can be reversed.

There are two ideas in play here:
a) There is a clear-cut notion of relativistic causality. The future
light-cone is distinct from the past light-cone.
b) The F=ma law certainly includes non-relativistic cases, and indeed
most of the discussion has concerned the notion that "F causes ma
(and not vice versa)" in non-relativistic situations.

We can certainly have both conversations, but we should be careful
to keep track of which is which.

In common usage, however, causality does not seem to
strongly require a temporal separation of the cause and resulting effect.

That's overstated. According to
http://en.wikipedia.org/wiki/Causality

"A neutral definition is notoriously hard to provide ...."
but
"It is usually presumed that the cause chronologically precedes the effect. "
and
"... causes must precede their effects temporally."


A ball bounces off a wall. What is the "cause" of that bouncing motion? If
the wall was absent, the ball would continue its motion. But the force on
the ball and the acceleration of the ball occur simultaneously. This is
where I have a hard time with the concept of "cause". I must admit that I
have not resolved this comfortably in my own mind. I would appreciate any
comments from the group on this particular example.

For the last ~400 years the recommended way out of this situation is to
duck the question of causation. We can *explain* the bounce and *describe*
the bounce and *calculate* the bounce ... all without discussing causation.

There are situations where we need to figure out the cause (e.g. Love
Canal) and situations where we don't (F=ma).