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Re: "Negotiating" a curve. EUREKA?



I think I found another misconception. An attempt to explain
the centripetal force (on the center of mass of a turning car) in
terms of an FBD (a free body diagram) is impossible, unless
the wheels of the car are excluded.

The implied assumption of any FBD is that we are dealing
with a particle, or with a rigid body. Each turning wheel is
a rigid object but it is not rigid with respect to the car. After
excluding the wheels (and the spinning axel, and the inner
bearing rings, and the balls) we do have a rigid body. The
forces acting on that body are at the outer rings of the ball
bearings, as pointed out by Bob, not at the contacts between
the tires and the road, as stated in so many textbooks.

Saying that the net force acting on the CM of a turning car
is the vector sum of forces by which the road acts on it is a
misconception, as far as the FBD is concerned. Indirectly
yes, directly no. Free body diagrams are limited to directly
applied forces. Why is this rule ignored when the topic is
described in textbooks? Do the authors want us to think in
terms tiny little whees of zero mass? That does not help to
elimiate conceptual difficulties.

Some may defned textbook writers by saying that every
thing we teach is only an approcimation. True. But not
every elimination of a feature is a simplification. Hiding
an essential element is an error not a simplification. It
makes the problem more difficult, not mor simple.

Ludwik Kowalski

To understand is to find a satisfactory causal relation.
To explain is to express understanding.
To teach is to promote it.