Re: "Negotiating" a curve. EUREKA?

[brian whatcott <inet@INTELLISYS.NET>]

At 15:37 11/7/99 -0500, Ludwik wrote:

> ...Why
> does the direction of the force, with which the road acts on
> the front wheel, change with the orientation of the wheel?
...
> Yes, only a physics teacher can ask questions like this....

> Ludwik Kowalski

John Denker offered an equivalent model of a tricycle running
on 3 beads sliding smoothly on three curved wires.
 This did not by any means assuage Ludwik's concern.

The short answer to why the force from the tire (or bead) changes
direction in a turn is: "because it must!"

Let me try using another model to illustrate.
A vehicle running straight is subject to tire friction, bearing
friction and air friction etc. These act opposite the velocity.

 A vehicle turning in a circle is subject to a new force.
It is a force transmitted inwards towards the center of the circle
via the tires. All the old forces still apply in the same old
directions. But this is a new force, acting in a new direction.

It is functionally similar to three strings which tether a
tricycle (and fastened to its wheels) to the center of an ice
skating rink, and apply sufficient tension to enforce its circular
 motion. Depending on the fore/aft position of the center of gravity
 of the tricycle (approximately speaking) the tension of the three
 strings may not be the same.

These strings provide a centripetal force.
Like the three tire contact patches each provide a centripetal
force. The friction which constitutes this centripetal force does
not act in the direction of the instantaneous velocity. It acts
towards the center of the turning circle (Approximately).
 That the frictional force can act in any
direction seems to be the camel which Ludwik will not swallow.

If he can accept that for a hypothetical tricycle running straight
on ice there would be no tension from strings run at right
 angles to its motion but there would appear a tension in strings
which act at right angles to its *turning* motion (the 'new' force),
 then I think Ludwik would have an acceptable explanation for students.
 Moreover, I think this could be diagrammed without distress.

After all, *I* can understand my explanation!   :-)

Sincerely


brian whatcott <inet@intellisys.net>
Altus OK