Answers to STUDY BREAK Questions
Essentials 5th
Chapter 10
TIDES
1. How is a forced wave different from a free wave?
Tide waves are called forced waves because they are never free of the forces that
cause them. In contrast, after they are formed, wind waves, seiches, and tsunami are free
waves -- they are no longer being acted upon by the force that created them and they do not
require a maintaining force to keep them in motion.
2. What celestial bodies are most important in determining tides?
The position and proximity of the moon makes the most important contribution to
tidal patterns. The sun’s influence on the tides is only 46% that of the moon’s.
3. In general terms, how is the pull of gravity between two bodies related to their distance?
The pull of gravity between two bodies is proportional to the masses of the bodies but
inversely proportional to the square of the distance between them.
4. What is a tractive force? How is it generated?
The combined outward-flinging force of inertia and inward-pulling force of gravity
are called tractive forces. Gravity and inertia don’t always act in exactly the same balanced
way on each particle of Earth and moon; the tractives forces are the net strength and direction
that result when the two forces are combined (Figure 10.5). The key to understanding tides is
to imagine Earth turning beneath bulges of water formed by tractive forces (as in Figure
10.7).
5. What body generates the strongest tractive forces?
Because of its proximity to Earth, the moon generates the strongest tractive forces.
6. What is a spring tide? A neap tide?
Spring tides occur when Earth, moon, and sun are aligned (Figure 10.11a). Neap
tides occur when Earth, moon, and sun form a right angle (Figure 10.11b).
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Answers to STUDY BREAK Questions
Essentials 5th
7. How does the equilibrium theory of tides differ from the dynamic theory?
The dynamic theory correctly treats tide waves as shallow-water waves. As Earth
turns, landmasses divert, slow, and otherwise complicate the movements of tidal crests. This
interference produces different patterns in the arrival of tidal crests at different places.
8. Are tides always shallow-water waves? Are they ever in “deep” water?
Because of their immense wavelength, tides can never be in “deep” water (that is,
water deeper than half the wavelength), even though their crests may traverse abyssal depths.
9. What tidal patterns are observed on the world’s coasts?
Some coastlines experience semidiurnal (twice daily) tides: two high tides and two
low tides of nearly equal level each lunar day. Others have diurnal (daily) tides: one high and
one low. Coastlines with mixed (or semidiurnal mixed) tides have successive high tides or
low tides of significantly different heights, caused by blending of diurnal and semidiurnal
tides. Figure 10.13 shows an example of each tidal pattern.
10. Are there tides in the open ocean?
Tidal crests rotate around amphidromic points – “no tide” points in the open ocean
(Figure 10.15). Because of the shape and placement of land masses around ocean basins, the
tidal crests and troughs cancel each other at these points.
11. How does basin shape influence tidal activity?
The largest tidal ranges occur at the edges of the largest ocean basins, especially in
bays or inlets that concentrate tidal energy because of their shape. If the basin is narrow and
restricted, the tide wave crest cannot rotate around an amphidromic point and simply moves
into and out of the bay. In other cases, arriving tide crests stimulate natural oscillation
periods of around 12 or 24 hours, resulting in extreme tides.
12. What’s a tidal bore?
A tidal bore is a steep wave moving upstream generated by the action of the tide crest
in the enclosed area of a river mouth.
13. What is a meteorological tide?
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Answers to STUDY BREAK Questions
Essentials 5th
Meteorological tides are weather-related alterations to predicted tidal cycles, such as
those associated with the storm surge of tropical cyclones.
14. Can astronomical and meteorological tides interact?
Arrival of a storm surge on top of a high tide can be especially devastating to coastal
regions. Some of the astonishing destructiveness of Hurricane Katrina in 2005 can be
attributed to the arrival of the surge (and wind-driven masses of water) coincidentally with a
high tide.
15. Distinct zones of marine organisms can usually be seen along rocky shores. How might
tidal patterns result in this sort of differential growth?
Within the intertidal zone, organisms are exposed to varying amounts of emergence
and submergence. The animals and plants sort themselves into horizontal bands based on
the amount of exposure they can tolerate. Each distinct zone is an aggregation of animals
and plants best adapted to the conditions within a particular narrow habitat.
16. Where is electrical power being generated from tidal movement?
There are major tidal power stations in France on the estuary of the river Rance and
on the Annapolis River in Nova Scotia.
17. Why isn’t tidal power being developed more aggressively?
Tidal power plants can be damaged by storms and corroded by seawater. Computer
simulations have suggested that installing a dam would change the resonance modes of a bay
or estuary—and therefore the height of the tide wave. Studies also suggest that sensitive
planktonic and benthic marine life would be disrupted.
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