Transcript Chapter 10: Tides - Washington University in St. Louis

Tides, Gravity, Centripetal force
Fig. 10-6
Fig. 10-7
Tide-producing forces
Gravity and
motions among
Earth, Moon,
and Sun.
Barycenter, and
centroid

Fig. 10-2
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Centripetal force
“tethers” Moon
to Earth
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Directed away
from barycenter
Fig. 10-4 a,b
Resultant tidal forces
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Fig. 10-6
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Fig. 10-7
Gravitational
force, Earth and
Moon
Centripetal force,
Earth and Moon
Resultant force
moves ocean
water horizontally
Tidal bulges
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Two equal and
opposite tidal
bulges
Earth rotates
beneath tidal
bulges
Two high tides
Two low tides
Per day
Fig. 10-8
Complications to simplest
equilibrium theory
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Oceans do not cover entire Earth
Oceans do not have uniform depth
Friction between ocean and seafloor
Continents
Moon not always in same place with
respect to Earth
Lunar day longer than solar day
Lunar day
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Moon revolves around Earth
Earth has to “catch up” with Moon to
reach same position
Fig. 10-9
Time between successive high
tides shifts day after day
 Moon rises later each
successive night
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Solar tidal bulges
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Tide-producing force of Sun less
than half of Moon’s since Sun
much farther away
Month tidal cycle

Spring tides
New Moon, Full
Moon
 Earth, Moon,
Sun align
(syzygy)
 Higher than
usual high tidesFig. 10-12
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Neap tide
First Quarter,
Last Quarter
 Earth, Moon,
Sun quadrature
 Lower than
usual high tide
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Fig. 10-12
Declination of Sun and Moon

Orientation of Sun, Moon to Earth’s
equator
Sun 23.5o N and S, yearly cycle
o
 Moon 28.5 N and S, monthly cycle
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Unequal tides
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Successive tides different tidal range
Unequal tidal range
Fig. 10-15
Elliptical orbits

Perigee
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Lunar tidal force
greater
Higher high tides
Apogee
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Lunar tidal force
lesser
Lower high tides
Fig. 10-16
Dynamic theory of tides
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Tide shallow-water wave
Speed varies with depth
 Lags behind Earth’s rotation
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Rotary flow in open ocean basins
Amphidromic point (‘panning for gold’)
 Cotidal lines
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Rotary flow
Crest (high tide) rotates
 Counterclockwise in Northern
Hemisphere
 Clockwise in Southern
Hemisphere
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Tidal patterns
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Diurnal
One high, one low tide per lunar day
 Period of tidal cycle 24 hours 50 minutes
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Semidiurnal
Two high, two low tides per lunar day
 Period 12 hours 25 minutes
 Equal range
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Mixed
Two high, two low tides per lunar
day
 Unequal range
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Most tides are mixed
Standing waves
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Forced standing
wave caused by
tides
Free-standing
waves caused by
strong winds or
seismic
disturbances
Fig. 10-22
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Node maximum
horizontal flow
Antinode
maximum
vertical flow
Fig. 10-23
Bay of Fundy
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Fig. 10-24
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Largest tidal range
(spring tide max 17
m)
Shape of basin
Oscillation period
close to tidal period
Shoals and narrows
to north
Basin oriented
toward right
(Coriolis moves
water toward right)
Tidal bores
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Wave created by
tide rushes
upstream
Large tidal range
Low-lying coastal
river
Max 8 m = more
than 25 feet high