Transcript Chapter 9: Tides

CHAPTER 9
Tides
Overview
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Rhythmic rise and fall of sea level
Very long and regular shallow-water
waves
Caused by gravitational attraction of Sun,
Moon, and Earth
Weight
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A measure of the pull of gravity
Gravity
The force of attraction between two objects.
The amount of force is determined by
1. The mass of the objects. The more
massive the object the greater the force.
2. The distance between the objects. The
greater the distance the less the force.
Gravity – Earth, Sun, & Moon
Which is more massive?
Sun
Which is closer to the Earth?
Moon
Which has the greater gravitational effect on
the earth?
Moon
Assumptions
The Earth is smooth (like a ball) – no
continents.
Water uniformly covers the Earth.
Gravitational forces
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Every particle attracts every other particle
Gravitational force proportional to product of
masses
Inversely proportional to square of separation
distance
Fig. 9.2
Earth’s Tidal Bulge
The Moon’s gravity pulls the
water out to one side.
Centrifugal Force
Centrifugal force is a force outward from the
center of rotation
Center of Mutual Orbit
If two objects have the same mass they will orbit
about a point half way between them.
This point is called the
Center of Mass
Center of Gravity
or
Barycenter
Earth’s Barycenter
The Earth/Moon center of orbit is a point inside the
surface of the Earth. But not the center of the Earth.
It is the point of mutual orbit due to gravity and motion.
Earth/Moon Motion
This is
what the
Earth/Moon
orbit looks
like.
Notice how
the Earth
“Wobbles”.
Equilibrium Bulge
The equilibrium bulge on the opposite side of
the Earth from the Moon is caused by
centrifugal force from the Earth’s Wobble.
The Tide is a Wave
The tide is a wave and it has two crests and two
troughs.
The crest is high tide and the trough is low tide.
Trough
Low Tide
High Tide
High Tide
Crest
Crest
Trough Low Tide
Tidal bulges (lunar)
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Small
horizontal
forces push
seawater into
two bulges
Opposite
sides of Earth
One bulge faces
Moon
Other bulge
opposite side
Fig. 9.6
Earth
Tidal Day (Lunar Day)
It take 24 hours for the Earth to make one rotation.
But the Moon has moved.
It takes an additional 50 minutes to get back to the
Moon
Lunar Day
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Moon orbits Earth
24 hours 50 minutes for observer to see
subsequent Moons directly overhead
High tides are 12 hours and 25 minutes apart
Fig. 9.7
Tidal bulges (lunar)
Moon closer to Earth so lunar tide-producing
force greater than that of Sun
Tidal bulges (solar)
Similar to lunar bulges but much smaller
Moon closer to Earth
Spring Tide
Sun & Moon Pull Together – New & Full Moon
Greatest tidal range (Distance from high to
low tide)
Highest High Tide & Lowest Low Tide
Time between spring tides about two weeks
Neap Tide
Sun & Moon Pull At 90o – Quarter Moons
Least tidal range (Distance from High to
Low tide)
Lowest High Tide & Highest Low Tide
Idealized tide prediction
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Two high tides/two low tides per lunar day
Six lunar hours between high and low tides
Complicating factors: Declination
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Angular distance of the Moon or Sun above or below
Earth’s equator
Sun to Earth: 23.5o N or S of equator
Moon to Earth: 28.5o N or S of equator
 Shifts lunar and
solar bulges away
from the equator
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Unequal tides
Fig. 9.11
Declination and tides
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Unequal tides (unequal tidal ranges)
Fig. 9.13
Complicating factors: Elliptical Orbits
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Tidal range greatest at perihelion (January) and
perigee
Tidal range least at aphelion (July) and apogee
Perigee and apogee cycle 27.5 days
Fig. 9.12
Real tides
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Earth not covered completely by ocean
Continents and friction with seafloor
modify tidal bulges
Tides are shallow water waves with speed
determined by depth of water
Tidal bulges cannot form (too slow)
Tidal cells rotate around amphidromic
point
Tidal cells in world ocean
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Cotidal lines
Tide wave rotates once in 12 hours
 Counterclockwise in Northern Hemisphere
Fig. 9.14
Tidal patterns
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Diurnal – Gulf Coast
 One high tide/one low tide per day
Semidiurnal – East Coast (Virginia Beach)
 Two high tides/two low tides per day
 Tidal range about same (same height)
Mixed - West Coast
 Two high tides/two low tides per day
 Tidal range different (not same height)
 Most common
Tides in coastal waters
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Standing waves
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Tide waves reflected by coast
Amplification of tidal range
Example, Bay of Fundy maximum tidal range 17 m (56 ft)
Tides in
coastal
waters
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Tidal bore
in lowgradient
rivers
Fig. 9A
Tidal Bore
Surfing
http://www.youtube.com/watch?v=4ZuZiLuHM1A
http://www.youtube.com/watch?v=mrNcZUrgCrM
http://video.google.com/videoplay?docid=713809622642491779&ei=92iS7PrFpKBlgeKkfSODQ&q=tidal+bore+video&hl=en#
Wave
http://www.youtube.com/watch?v=ghBPn3UDt9ghttp://www.youtube.com/watch?v=ghBPn3
UDt9g
http://www.youtube.com/watch?v=1HMCK6wPg8E
Coastal Tidal Currents
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Reversing current
High tide, Flood tide,
Flood current
seawater moves on
shore
Low tide, Ebb tide,
Ebb current seawater
moves offshore
High velocity flow in
restricted channels
Coastal tidal
currents
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Whirlpool
 Rapidly spinning
seawater
 Restricted
channel
connecting two
basins with
different tidal
cycles
Fig. 9.19
Tides and marine life
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Fig. 9C
Tide pools and life
Grunion spawning
Tide-generated power
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Renewable
resource
Does not
produce power
on demand
Possible
harmful
environmental
effects
Most devices
use tidal
currents
End of CHAPTER 9
Tides