Re: Crocodiles problem.

[Michael Bowen <fizzbowen@MINDSPRING.COM>]

At 10:43 2001/10/18, Ludwik Kowalski wrote:

> So the puzzling question has to do with the hint. Is ot okay
> to associate potential energy with the a noncoservative force?

Why is the force nonconservative?

One of the many ways to spot a conservative force is to note that its value
depends only on the position (x, y, z) of the object on which it acts. Any
constant force technically satisfies this criterion. The "flat-earth
approximation" F = mg for the gravitational force is such an example.

In "real life", of course, air resistance is generally nonconservative; it
depends on velocity, the orientation of the object, and so on. However, as
the problem is stated, we are to ignore these complications and assume F =
constant. With that proviso, air resistance suddenly becomes conservative;
there is no magic involved.

Granted, this is not physical, but to me it seems no worse than other
idealizations commonly made in introductory physics, such as ignoring air
resistance completely in beginning kinematics, or using frictionless
pulleys, massless strings, and ideal springs in beginning dynamics.

For all intents and purposes, there are only two conservative forces in a
first-semester course: gravity and the elastic force associated with
springs. (Yes, the electrostatic force is conservative, but that's
second-semester for most of us.) After fifty-odd problems, I suspect
Serway, et al, got tired of inventing PE problems involving only gravity
and springs, or perhaps they just wanted to remind students that these
aren't the only two types of forces in the universe. Granted, they are
stretching the limits of physical "believability" by stating the
constant-force assumption for the wind. But it does give students an
opportunity to "solve" a PE problem involving something besides gravity and
springs.

--MB