> When you start walking from a stationary
> position, what force is acting in the direction
> of your acceleration?
I am pushing the road back and the road acts
on me with an equal and opposite static friction.
In this case the force causing the acceleration
is indeed equal to mu*m*g. Likewise, in the
case of a "powered wheel" the wheel pushes
the road (cause) and the road reacts on the
car with F=mu*m*g.
And, in my opinion, the wheels which are not
"powered" do not contribute to the net F.
My question was about the centripetal force.
Here is one possible approach. We place
ourselves in a rotational frame of reference.
In that frame the centrifugal "action" force is
directed away from the center. And it is real.
The car acts on the road with the centrifugal
force m*v^2/r and the road acts on the car
with a force equal in magnitude but opposite
in direction. The lager the m*v^2/r becomes
(up to a limit) the larger is the mu*m*g (to
sustain the constrain imposed v and r).
That kind of reasoning "naturally" leads to the
m*v^2/r=mu*m*g relation. The static friction
force becomes a responding force, as it should
be. I suspect, however, that many will object
to the idea of explaining things in terms of
centrifugal forces. The textbook I am using has
a provocative conceptual question: "Explain
why Earth is not spherical in shape, but bulges
at the equator." What answer is expected?
Ludwik Kowalski