In a message dated 96-07-15 13:30:09 EDT, you write:
<< To Robert Carlson and and all who think like him, Dear Robert:
I must disagree that F = m a is just a definition. Of course, here
we may slip into deeper philosophy: is velocity just a definition?
Acceleration?>>
Emilio,
What I said was, and I quote from my own message:
<In first approaching force, I make it clear that Fnet = ma is a definition.
It is something we are all going to agree on.>
Is this not the meaning of net force? Now a quote from Webster's dictionary:
Definition: An explanation of what a word, etc. means.
When we write Fnet = ma are we not defining that when we see a mass, m,
accelerating with an accleration a, then there must be a non-zero net force
causing this acceleration? And also, if the mass has zero acceleration, then
the net force is also zero? Doesn't this last statement simply follow from
the definition (meaning) of Fnet?
Is velocity a definition? Yes it is. The meaning, definition, for the
average velocity an object has in the x direction is defined as: Vave =
delta(x)/delta(t). Is this not the meaning (definition)? Similarly, the
equation for acceleration also defines acceleration. Does it not tell us
what acceleration means?
Emilio, then you say:
<<The fact is that forces DO exist even in the absence
of motion . . . >>
I said nothing to the contrary. What I said was:
<Using Fnet = ma, I stress that whatever direction the acceleration is in,
this is the same direction the net force must be in. Why? Because we all
agree on the definition. (Please, no negative mass here.) I also stress
that not all the forces acting on the object must be in the direction of the
acceleration. And, it is the physicist's job to identify which forces are
important and in which directions they are acting on the object.>
If a is zero, then Fnet is zero too. Is this not true? However, I did not
say that because Fnet is zero that there aren't any forces acting on the
mass. This may be the case (at least mathematically), but does not have to
be the case. Fnet = 0 simply means one of two possible situations:
1. There really are no forces acting on the object (I can't think of a real
situation here, at least in our universe).
or
2. All the vector forces acting on the object sum to zero.
Emilio, please read what I wrote more carefully, or at least quote my message
as you comment on it.