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Re: Hot air rising and automobile thermometers

On Mon, 26 Jul 1999, Leigh Palmer wrote:

But Leigh, surely a steady wind experiences more than one force. While the
net-force should be zero, the current question really addresses the
'driving' force, the one that keeps the air moving in spite of the
'frictional-type' forces which must also be present. Indeed, if there were
no such driving force the motion of the air would soon cease.

There is more than one force acting, but there is no frictional force.
Given a piece of air in a steady wind the immediately adjacent air is
moving at the same speed. By "to first order" I mean considering no
wind shear. The conventional explanation of geostrophic winds is of
first order. In this case the net force is zero. The force due to the
horizontal component of the pressure gradient is exactly balanced by
the horizontal component of the Coriolis force. Of course more forces
are present in real situations, but those are the two large forces.

When a skater brings his/her arms in and spins faster as a result, is it
proper to say that the spin is driven by the inward force exerted by the
skater on his/her arms? Before this discussion started, I would've said
that it was proper. Since I see the geostrophic winds as being similar
and I also thought it proper to say that the pressure gradient was driving
the winds.

However, upon second thoughts, I think it might be *more* proper to say
that the winds were "initially" driven by the pressure gradient force.

Perhaps it might be worthwhile to consider the following situation: if
there exists only a x-dir force A, an object on the earth will speed up
and eventually reach a constant speed in a direction perpendicular to the
force at a constant speed. The speed is constant and the net force is
zero (force A balanced by coriolis force). If the x-dir force suddenly
disappears, the object would then start to follow a circular motion (only
the coriolis force would be present). It seems weird to say that initial
force A (which is no longer present) is still driving it.

Does this sound better?

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| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
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