From: Ron Kean (ronkean@juno.com)
Date: Mon Jul 12 1999 - 09:50:54 MDT
On Mon, 12 Jul 1999 13:23:50 +0100 Rob Harris Cen-IT
<Rob.Harris@bournemouth.gov.uk> writes:
[Ron Kean]
>> Gravity waves are a different phenomenon than gravitational waves.
>> Gravity waves are waves
>> consisting of the motion of fluid matter in a gravitational field.
>The
>> ripples on the surface of a pond are called gravity waves.
>>
> Are you certain about this? I don't remember any of this when
>I did
>A level Physics. I would call waves in water exactly that: water
>waves,
>sound = air waves, light = wave in some kind of electromagnetic ether
>(spacetime?) - I don't know this part at all - anyone got the info?
>
> Rob.
>
>
Certainly surface waves in water can be called water waves, and usually
are (in a non-technical context), without any misunderstanding. But
sound travelling through water can also be called 'waves in water', so
the term 'water waves' is technically imprecise, or ambiguous. So the
waves on the surface of a pond are better called 'gravity waves along a
water-air fluid interface', or just 'gravity waves' for short, since
their propagation depends on the interaction between the earth's gravity
(which is perpendicular to the surface of the pond) and the density,
viscosity, and surface tension of the water. Without gravity those waves
won't happen. That's why they're called 'gravity waves', as a technical
term.
Reference: Van Nostrand's Scientific Encyclopedia, Sixth Edition, 1983,
ISBN 0-442-25161-0, Lib. of Congress Cat. No. 82-4936. Page 267, in the
'Atmosphere' article, concerning atmospheric gravity waves. Quote:
Gravity wave disturbances are those in which buoyancy (or reduced
gravity) acts as the restoring force on parcels displaced from
hydrostatic equilibrium. There is a direct oscillator conversion between
potential and kinetic energy in the wave motion. Close quote.
Sound waves, on the other hand, can travel through liquid water quite
nicely in the complete absence of external gravity, so long as the water
is pressurized. They are not gravity waves, but rather are acoustic
waves. Acoustic waves can travel through the bulk of a solid elastic
medium. With a solid medium the propagation is determined by the density
and elastic modulus (stiffness) of the solid medium. Acoustic waves can
also travel along the surface of a solid elastic medium, rather than
through its bulk. They are called surface acoustic waves, or SAWs for
short. They are not much affected by gravity, although I suppose they
could be disrupted by an extremely strong gravitational field. Light is
also affected by gravity, but only slightly unless the gravitational
field is extremely strong.
Gravitational waves are produced when matter accelerates or vibrates.
Those waves are thought to propagate through space at the speed of light,
but they are not light, and they are not electromagnetic waves.
Gravitational waves have sometimes been called gravity waves, but that is
incorrect terminology.
Ron Kean
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