#### NCERT Solutions for Class 11th: Ch 14 Movements of Ocean Water

Exercises

Page No: 125

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
►(a) tide

(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.
►(a) As result of the moon and the sun pulling the earth gravitationally in the same direction.

(iii) The distance between the earth and the moon is minimum when the moon is in :
(a) Aphelion
(b) Perigee
(c) Perihelion
(d) Apogee
► (b) Perigee

(iv) The earth reaches its perihelion in:
(a) October
(b) September
(c) July
(d) January
► (d) January

(i) What are waves?

Waves are energy, not the water which moves across the ocean surface. It generated by wind driving against water.

(ii) Where do waves in the ocean get their energy from?

Wind provides energy to the waves. Wind causes waves to travel in the ocean and the energy is released on shorelines.

(iii) What are tides?

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?

The tides are caused by the gravitational pull of the sun and the moon and the centrifugal force

(v) How are tides related to navigation?

Tides 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.

(i) How do currents affect the temperature? How does it affect the temperature of coastal areas in the N. W. Europe?

The currents directly affect the temperature. The temperature varies depending on whether currents are warm or cold:
• Cold currents bring cold water into warm water areas. These currents are usually found on the west coast of the continents in the low and middle latitudes (true in both hemispheres) and on the east coast in the higher latitudes in the Northern Hemisphere
• 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.

In North West Europe, warm currents are present which increase the temperature in coastal areas in the North West Europe.

(ii) What are the causes of currents?

Oceans currents are caused by two forces mainly:
(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:

• Heating by solar energy:  Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope.

• Wind: It blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course

• Gravity: It tends to pull the water down the pile and create gradient variation.

• Coriolis force: This intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere. These large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins.

The secondary forces that influence the currents are:

• Difference in water density: It affect vertical mobility of ocean currents. Denser water tends to sink, while relatively lighter water tends to rise.

• Difference in Salinity: Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water.

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