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Re: Car acceleration



Many responses, I think this one os closest to what is actually happening.
Here is my take:

From a FBD, it appears as though the friction between the car and the road
provides the force to accelerate the car. But what is that friction? It
is not kinetic because these surfaces are not sliding. It cannot be
static, because static can do no work - yet the car gains kinetic energy as
a result of that force. It must be something alse, possibly a quasi-static
friction. But where does that come from? Is it due to the flexing of the
tire?
What happens then if you take a rigid tire, with a small footprint and only
static friction. Could you accelerate with this?

Another option I looked at - consider the wheel/axle as one system and
the 'car' as another. There are three pairs of forces, the friction
between the tire and the ground, the contact in the axle housing and
the 'drive - force' from the engine/drive shaft.
Suppose the whole thing is going to move off to the right, with the wheel
rotating CW. The friction will produce a CCW torque on the wheel, the axle
housing (assume frictionless) will produce no torque, so the drive must
produce a CW torque. That will produce a CCW torque on the car - balanced
presumably by the normal on the rear wheels.
If the drive and friction push the wheel to the right and the housing to
the left, it is still possible to produce a force on the car to the right
as well. This force will act over a distance - and will do work. The
force on the wheel will act over a distance and will do work.

Now, is that sufficient? Is the car going to move? I have relied on
static friction only here and the end result is that the force is between
the engine and tires.

[The analogy of a skater pushing off a wall is not completely valid - that
force is exerted over a finite time and hence a finite distance.]