IMHO best answer is none of the above (if we're
striving for clarity with our students). A force
diagram would show a drag force (mainly rolling
friction) opposing motion and the force of the road
pushing forward on the car. This latter force is an N3
reaction to the rearward push of the tires on the road
which is only possible if the surface is not
frictionless. This is the same process that allows us
to walk. John Barrere
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