Re: [Phys-l] circular motion FBD

[John Denker <jsd@av8n.com>]

On 10/31/2011 04:57 PM, Anthony Lapinski wrote:

> Also, doesn't static friction propel cars and bikes (and people walking)
> forward? If you stop pedalling or stepping on the gas, the moving vehicle
> will eventually stop.

Well ... as always, it helps to distinguish the /data/ from the
/interpretation/.  Both are important, but they're not the same
thing.

The data is important, but we don't need to discuss it, because
we all know what will happen.

The interpretation is important, and needs to be discussed, because
we have a choice.
  a) Aristotle said an object at rest tends to remain at rest, and
   an object in motion tends to come to rest.  According to this
   model, this is the natural "baseline" behavior and does not need
   to be explained.
  b) Galileo and to some extent Newton came up with the idea that a
   body at rest tends to remain at rest, and an object in motion tends
   to remain in motion.  This is the standard post-1632 physics model.
   According to this model, continued motion is the natural "baseline"
   behavior, and need not be explained, and indeed cannot be explained.

   In contrast, the slowing-down of the car needs to be explained.
   It can be explained in terms of aerodynamic drag, internal friction
   associated with the flexing of the tires, et cetera.

===========

To repeat:  We do not get to choose the data, but we do get to choose
the model.  I strongly recommend choosing the model that does not
attempt to explain why the car keeps going.  Instead, the model explains 
the various physical processes that are associated with slowing down.
This in turn explains the need for some nonzero thrust to overcome
drag if we don't want the car to slow down.

There are about a dozen practical reasons and also pedagogical reasons
why the post-1632 Galileo/Newton model works better.