3. Tides

Do the photographs (Fig 3.1a and 3.1b) show the same place or different places?
The photographs show the same place.

Observe and note the spread of water seen in both the photographs.
In figure 3.1a, the sea water is spread very close to the coast. In figure 3.1b, the sea water has receded and is far away from the coast.

What is this natural event called?
This natural event is called Tides (High tide and Low tide).

In which direction did the piece of chalk fall?
The piece of chalk fell outwards, away from the centre of the notebook.

Where did the water in the glass show a bulge?
The water in the glass showed a bulge towards the outer edge of the glass.

What effect did the movement have on the things attached to the keyring?
The things attached to the keyring flew outwards, away from the finger holding it.

What happened to the water in the container and the mixer?
The water bulged and moved outwards towards the walls of the container and mixer.

Which forces could be operating in activities listed above?
The Centrifugal force and Gravitational force are operating in these activities.

In which activities did you find that the centrifugal or gravitational force was greater?
In all the spinning activities, the centrifugal force was greater locally, pushing the objects outward and overcoming the gravitational pull holding them in place.

Large rockets are used to go into the space away from the earth. Which force do they act against?
Large rockets act against the Gravitational force of the earth.

(1) Tides are influenced more by the moon than the sun.
The sun is much larger than the moon, but it is located far away from the earth. The moon is much closer to the earth. Therefore, the gravitational pull of the moon is more effective and plays a dominant role in causing tides on the earth.

(2) At some places along the coast, the low lying areas turn into lagoons or marshy lands.
During high tides, sea water rushes into the low-lying areas near the coast. Over time, sediments brought by the sea accumulate, and the trapped sea water creates shallow water bodies. This continuous process turns these low-lying areas into lagoons or marshy lands, which also become suitable for mangrove growth.

(3) Place located on the opposite meridian to the place experiencing high or low tide will also experience high or low tide respectively.
When a place faces the moon, it experiences a high tide due to the moon's gravitational pull. At the exact same time, the place on the opposite meridian experiences a high tide as well. This happens because the centrifugal force generated by the earth's rotation pushes the water outward, opposite to the direction of gravity. Consequently, the places at right angles to these meridians experience low tide.

(1) If there is high tide at 7 am, find the timings of the next high and low tides on the same day at a given place.
The time difference between two consecutive high tides is 12 hours and 25 minutes. Therefore, if there is a high tide at 7:00 am, the next high tide will occur at 7:25 pm.
A low tide occurs roughly halfway between two high tides (approx. 6 hours 12.5 minutes later). Therefore, the next low tide will occur around 1:12 pm.

(2) If at Mumbai (73° E meridian), there is high tide at 1.00 pm on Thursday, then on which other meridian will there be high tide too? State with reasons.
There will be a high tide on the 107° W meridian.
Reason: A place located on the exact opposite meridian always experiences a high tide at the same time due to the earth's centrifugal force. The opposite meridian to 73° E is calculated as (180° - 73°) = 107° W.

(3) Explain the reasons for the generation of waves.
Waves are primarily generated by the force of the wind pushing against the surface of the sea water, causing it to move up and down. Occasionally, destructive waves are also generated by underwater earthquakes or volcanic eruptions occurring below the ocean floor, which are known as tsunamis.

(1) Swimming
Swimmers must know the tide timings. The water level and currents change quickly with the tides, so swimming at the wrong time can lead to drowning.

(2) Steering a ship
During high tide, the water level near the coast increases. Ships wait for the high tide so they can safely steer and move into shallow ports without running aground.

(3) Fishing
During high tide, a large amount of sea water enters the creeks along with fish. Fishermen depend on high tides to catch fish easily in these areas.

(4) Salt pans
During high tide, the sea water floods the coastal areas and can be collected and stored in salt pans. This trapped water is then evaporated by the sun to obtain salt.

(5) Going to coastal areas for trips
Tourists going to coastal areas must be aware of tide timings. Exploring beaches or rocky patches without knowing when the high tide will return can lead to visitors getting trapped or washed away.

(1) Which phase of the moon does it show?
It shows the Quarter phase of the moon (First or Third quarter).

(2) What are the relative positions of the moon, the sun and the earth?
The moon, the earth, and the sun are positioned in such a way that the lines joining the earth to the sun and the earth to the moon make a right angle (90°).

(3) What effect will it have on the tides?
Because the forces of the sun and the moon operate at right angles to each other, they do not complement each other. This results in a Neap Tide, where the high tide is lower than usual, and the low tide is higher than usual.

(1) High tide and low tide.

(2) Spring tide and neap tide.