During a recent, lengthy, private discussion amongst a few of us, I
started out in the Fnet causes acceleration camp, but eventually came to
realize that forces do not just run around the universe by themselves,
causing things to accelerate.
A force (or that final bit of force that causes the net force to become
unbalanced) comes from some agent - either some piece of matter or a
passing EM field, and both the force *and* the acceleration of whatever
it interacts with happen together, with no cause and effect relationship.
I'm going to have to disagree with Bill, who argues that a net force
does not cause acceleration, but the individual forces do.
The argument was also made that using cause-effect language might lead
students to believe that there is some delay between force and
acceleration. I don't know if this has been studied, but since they
already do not distinguish between acceleration and velocity, I
certainly don't want to add any additional confusion.
It is sufficient to say, and essential for students to master, that when
something is accelerating, there is going to be an unbalanced force, and
vice versa. When you see one, look for the other.
Some people state the causality as "I apply a force, and then the object
accelerates." The flaw in such a statement is that there is no moment in
time when a force is applied. Every force producing piece of matter in
the universe is already interacting with every other piece of matter on
which it has an influence. In the case of gravity, that's everything. In
the case of the electric force that we usually associate with the
concept of "contact," it's the charged particles. The interactions are
already there, and on the small scale the only choice we have is to
strengthen or weaken them by adjusting the distance between them. These
influences have associated accelerations that have also been going on
since the particles were formed, and as the positions adjust, both the
forces and the accelerations adjust in tandem.
Your basic elliptical orbit is a stable, continuous variation of
position, force and acceleration.
To address the student who says "The car didn't start accelerating until
you pushed it," you would really have to get into the microscopic
behavior of the materials as they approach one another. In a
frictionless vacuum situation, you might see an initial attraction,
followed by the repulsion as "contact" is fully established. Both the
force and the acceleration are functions of distance, not each other.
(Think of the Cavendish apparatus.)
This is one of those situations where the eye should not be believed,
and why it took 2000 years to figure this stuff out once people really
started looking at it.
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It is often stated that physicists build models of reality. A modelwhich
states that acceleration is always caused by a net force seemsto be very
useful. That is what Sarma would say about the a=F/m. Themass of an
object, in a classical model, does not depend on speed. Butthe m must
known to calculate the a(t) when a particular F(t) isgiven. Do I
interpret him correctly?Yes, a mathematician can write the second law as
F=m*a, or m=F/a. Thatdoes not contradict the useful model of a
physicist--in order toaccelerate an object one needs a net force.
Causality is part of themodel, it is not part of reality.Yes, I know
that more general models can be, and have been, created.All models have
limited validity. The simplest model is appropriate inteaching
introductory physics courses. That is what most authors oftextbooks do.
The assumptions under which simple models are valid areusually clearly
stated. Learning about more general models, inadvanced classes, does not
mean "unlearning of what has already beenlearnt."