CHAPTER 9 Tides

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CHAPTER 9Tides
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Overview

• Rhythmic rise and fall of sea level
• Very long and regular shallow-water waves
• Caused by gravitational attraction of Sun, Moon,
  and Earth

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Weight

• A measure of the pull of gravity

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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.

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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

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Assumptions

• The Earth is smooth (like a ball) no
  continents.
• Water uniformly covers the Earth.

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Gravitational forces

• Every particle attracts every other particle
• Gravitational force proportional to product of
  masses
• Inversely proportional to square of separation
  distance

Fig. 9.2
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Earths Tidal Bulge
The Moons gravity pulls the water out to one
side.
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Centrifugal Force

• Centrifugal force is a force outward from the
  center of rotation

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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
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Earths 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.

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Earth/Moon Motion

• This is what the Earth/Moon orbit looks like.
• Notice how the Earth Wobbles.

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Equilibrium Bulge

• The equilibrium bulge on the opposite side of
  the Earth from the Moon is caused by centrifugal
  force from the Earths Wobble.

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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
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Tidal bulges (lunar)

• Small horizontal forces push seawater into two
  bulges
• Opposite sides of Earth
• One bulge faces Moon
• Other bulge opposite side Earth

Fig. 9.6
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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
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Lunar Day

• 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
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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
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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
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Neap Tide
Sun Moon Pull At 90o Quarter Moons
Least tidal range (Distance from High to Low
tide) Lowest High Tide Highest Low Tide
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Idealized tide prediction

• Two high tides/two low tides per lunar day
• Six lunar hours between high and low tides

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Complicating factors Declination

• Angular distance of the Moon or Sun above or
  below Earths 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
• Unequal tides

Fig. 9.11
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Declination and tides

• Unequal tides (unequal tidal ranges)

Fig. 9.13
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Complicating factors Elliptical Orbits

• 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
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Real tides

• 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

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Tidal cells in world ocean

• Cotidal lines
• Tide wave rotates once in 12 hours
• Counterclockwise in Northern Hemisphere

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Fig. 9.14
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Tidal patterns

• 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

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Tides in coastal waters

• Standing waves
• Tide waves reflected by coast
• Amplification of tidal range
• Example, Bay of Fundy maximum tidal range 17 m
  (56 ft)

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Tides in coastal waters

• Tidal bore in low-gradient rivers

Fig. 9A
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Tidal Bore

• Surfing
• http//www.youtube.com/watch?v4ZuZiLuHM1A
• http//www.youtube.com/watch?vmrNcZUrgCrM
• http//video.google.com/videoplay?docid7138096226
  42491779ei92-iS7PrFpKBlgeKkfSODQqtidalborevi
  deohlen
• Wave
• http//www.youtube.com/watch?vghBPn3UDt9ghttp//w
  ww.youtube.com/watch?vghBPn3UDt9g
• http//www.youtube.com/watch?v1HMCK6wPg8E

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Coastal Tidal Currents

• 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

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Coastal tidal currents

• Whirlpool
• Rapidly spinning seawater
• Restricted channel connecting two basins with
  different tidal cycles

Fig. 9.19
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Tides and marine life

• Tide pools and life
• Grunion spawning

Fig. 9C
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Tide-generated power

• Renewable resource
• Does not produce power on demand
• Possible harmful environmental effects
• Most devices use tidal currents

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End of CHAPTER 9 Tides