Tides Overview n n n Rhythmic rise and
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Tides
Overview n n n Rhythmic rise and fall of sea level Very long and regular shallow-water waves Caused by gravitational attraction of Sun, Moon, and Earth
Tide-generating forces n n Barycenter gravity
Barycenter n n n center of mass point at which system's mass behaves as if it were concentrated function only of the positions and masses of the particles that comprise the system
Barycenter between Moon and Earth n Mutual orbit due to gravity and motion n Fig. 9. 1
Newton’s law of Gravity
Gravitational forces n n n Every particle attracts every other particle Gravitational force proportional to product of masses Inversely proportional to square of separation distance Fig. 9. 2
Centripetal force n n n external force required to make a body follow a circular path at constant speed directed inward, toward the center of the circle it is a force requirement, not a particular kind of force.
Centripetal force n n Center-seeking force Tethers Earth and Moon to each other Fig. 9. 3
Tide-producing forces n n Resultant forces = differences between centripetal and gravitational forces Tide-generating forces are horizontal components Fig. 9. 4
Tidal bulges (lunar) n n Small horizontal forces push seawater into two bulges Opposite sides of Earth Fig. 9. 6
Tidal bulges (lunar) n Moon closer to Earth so lunar tideproducing force greater than that of Sun
Ideal Earth n n n covered by ocean Two tidal bulges Two high tides, 12 hours apart
High tide, flood tide n seawater moves on shore
Low tide, ebb tide n seawater moves offshore
Lunar Day n n n 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 (solar) n n Similar to lunar bulges but much smaller Moon closer to Earth
spring tide n New/full moon – tidal range greatest Time between spring tides about two weeks
neap tide n Quarter moons – tidal range least
Earth-Moon-Sun positions and spring and neap tides Fig. 9. 9
Other complicating factors: declination n n Angular distance Moon or Sun above or below Earth’s equator Sun to Earth: 23. 5 o N or S of equator Fig. 9. 11
Moon to Earth: 28. 5 o N or S of equator
declination n Shifts lunar and solar bulges from equator n Unequal tides
Declination and tides n Unequal tides (unequal tidal ranges) Fig. 9. 13
Other complicating factors: elliptical orbits n n n 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
Lunar day n n n 24 hours and 50 minutes moon revolves around the Earth in the same direction that the Earth is rotating on its axis it takes the Earth an extra 50 minutes to メcatch up to the moon
Lunar day n n Earth rotates through two tidal bulges every lunar day we experience two high and two low tides every 24 hours and 50 minutes
Idealized tide prediction n n Two high tides/two low tides per lunar day Six lunar hours between high and low tides
Real tides n n n 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
amphidromic point n n Tidal cells rotate around them occur because of the coriolis effect and interference with ocean basins
At the amphidromic point, there is almost no vertical movement.
Tidal cells in world ocean n n Cotidal lines Tide wave rotates once in 12 hours n Counterclockwise in Northern Hemisphere
Fig. 9. 14
Tidal patterns n n n Diurnal n One high tide/one low tide per day Semidiurnal n Two high tides/two low tides per day n Tidal range about same Mixed n Two high tides/two low tides per day n Tidal range different n Most common
Tides in coastal waters n n n Standing waves remains in a constant position result of interference between two waves travelling in opposite directions.
Standing wave n Tide waves reflected by coast n Amplification of tidal range n Example, Bay of Fundy maximum tidal range 17 m (56 ft)
Tides in coastal waters n Tidal bore in lowgradient rivers Fig. 9 A
Coastal tidal currents n Reversing current n Flood current n Ebb current n High velocity flow in restricted channels Fig. 9. 18
Coastal tidal currents n Whirlpool n Rapidly spinning seawater n Restricted channel connecting two basins with different tidal cycles Fig. 9. 19
Tides and marine life n n Fig. 9 C Tide pools and life Grunion spawning
Tide-generated power n n n Renewable resource Does not produce power on demand Possible harmful environmental effects
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