NCERT EXERCISES
1. Multiple choice questions.
(i) Upward and downward
movement of ocean water is known as the:
(a) Tide (b) Current (c) Wave (d) None
of the above
Ans. (c) Wave
(ii) Spring tides are
caused:
(a) As result of the moon and the
sun pulling the earth gravitationally in the same direction.
(b) As result of the moon and the
sun pulling the earth gravitationally in the opposite direction.
(c) Indention in the coast line.
(d) None of the above.
Ans. (a) As result of the moon
and the sun pulling the earth gravitationally in the same direction.
Note – The Spring tides occur
twice a month, one on full moon period and another during new moon period.
(iii) The distance between
the earth and the moon is minimum when the moon is in:
(a) Aphelion (b) Perigee (c) Perihelion (d) Apogee
Ans. (b) Perigee
(iv) The earth reaches its
perihelion in:
(a) October (b) September (c) July
(d) January
Ans. (d) January
2. Answer the following
questions in about 30 words.
(i) What are waves?
Ans. Waves are actually the
energy, not the water as such, which moves across the ocean surface. Water
particles only travel in a small circle as a wave passes. Wind provides energy
to the waves. Wind causes waves to travel in the ocean and the energy is
released on shorelines.
(ii) Where do waves in the
ocean get their energy from?
Ans. Most of the waves are
caused by the wind driving against water. When a breeze of two knots or less blows
over calm water, small ripples form and grow as the wind speed increases until
white caps appear in the breaking waves. Waves may travel thousands of km
before rolling ashore, breaking and dissolving as surf. The largest waves are
found in the open oceans. Waves continue to grow larger as they move and absorb
energy from the wind.
(iii) What are tides?
Ans. The periodical rise and
fall of the sea level, once or twice a day, mainly due to the attraction of the
sun and the moon, is called a tide.
(iv) How are tides caused?
Ans. The Moon’s
gravitational pull to a great extent and to a lesser extent the Sun’s
gravitational pull, are the major causes for the occurrence of tides.
Another factor is centrifugal force, which is the force that acts to
counter balance the gravity. Together, the gravitational pull and the
centrifugal force are responsible for creating the two major tidal bulges on
the earth. On the side of the earth facing the moon, a tidal bulge occurs
while on the opposite side though the gravitational attraction of the moon is
less as it is farther away, the centrifugal force causes tidal bulge on the
other side
(v) How are tides related
to navigation?
Ans. Since tides are caused by
the earth-moon-sun positions which are known accurately, the tides can be
predicted well in advance. This helps the navigators and fishermen plan their
activities. Tidal flows are of great importance in navigation. Tidal heights
are very important, especially harbours near rivers and within estuaries having
shallow ‘bars’ at the entrance, which prevent ships and boats from entering
into the harbour.
3. Answer the following
questions in about 150 words.
(i) How do currents affect
the temperature? How does it affect the temperature of coastal areas in the N.
W. Europe?
Ans. Ocean currents have a
number of direct and indirect influences on human activities. West coasts of
the continents in tropical and subtropical latitudes (except close to the
equator) are bordered by cool waters. Their average temperatures are relatively
low with a narrow diurnal and annual ranges. There is fog, but generally the
areas are arid. West coasts of the continents in the middle and higher
latitudes are bordered by warm waters which cause a distinct marine climate.
They are characterised by cool summers and relatively mild winters with a
narrow annual range of temperatures.
Warm currents flow parallel to the
east coasts of the continents in tropical and subtropical latitudes. This
results in warm and rainy climates. These areas lie in the western margins of
the subtropical anti-cyclones.
The mixing of warm and cold
currents help to replenish the oxygen and favour the growth of planktons,
the primary food for fish population. The best fishing grounds of the
world exist mainly in these mixing zones.
Warm currents bring warm
water into cold water areas and are usually observed on the east coast of
continents in the low and middle latitudes (true in both hemispheres). In the
northern hemisphere they are found on the west coasts of continents in high latitudes.
For example Gulf Stream affects the temperature of coastal areas in
the N. W. Europe.
(ii) What are the causes
of currents?
Ans. Ocean Currents - Ocean currents are like
river flow in oceans. They represent a regular volume of water in a definite
path and direction.
Ocean currents are
influenced by two types of forces namely:
(i)
Primary forces that initiate the movement of water;
(ii)
Secondary forces that influence the currents to
flow.
The primary forces that influence the currents are: (i) heating by solar energy; (ii) wind; (iii)
gravity; (iv) Coriolis force.