Chapter 3 - Tides
1. Prepare a chain by matching the following.
| Group A | Group B | Group C |
|---|---|---|
Waves | 8th phase of the moon (Quarter) | Objects get thrown towards the outer side. |
Centrifugal force | New moon day | Highest high tide occurs on this day. |
Gravitational force | Rotation of the earth | These are also generated due to earthquakes and volcanoes. |
Spring Tide | The moon, the sun and the earth | The forces of the sun and the moon operate in different directions. |
Neap Tide | Wind | Operates in the direction towards the centre of the earth. |
Ans:
| Group A | Group B | Group C |
|---|---|---|
Waves | Wind | These are also generated due to earthquakes and volcanoes. |
Centrifugal force | Rotation of the earth | Objects get thrown towards the outer side. |
Gravitational force | The moon, the sun and the earth | Operates in the direction towards the centre of the earth. |
Spring Tide | New moon day | Highest high tide occurs on this day. |
Neap Tide | 8th phase of the moon (Quarter) | The forces of the sun and the moon operate in different directions. |
2. Give geographical reasons:
(1) Tides are influenced more by the moon than the sun.
Ans:
(i) The gravitational pull of the moon, the sun, and the earth are responsible for the occurrence of the tide.
(ii) Compared to the sun, the moon is quite close to the earth.
(iii) As an effect, the gravitational force of the moon becomes more effective than that of the sun.
(iv) Therefore, tides are more influenced by the moon than the sun.
(2) At some places along the coast, the low lying areas turn into lagoons or marshy lands.
Ans:
(i) Low-lying areas along the coast are flooded and drained by the salt water brought in by the tides.
(ii) This process led to the gradual deposition of some seawater and sediments in the low-lying areas.
(iii) In such a region, plants then take roots and hold the soil firm against the force of tides and waves; this gradually increases biodiversity in the low-lying areas.
(iv) In this way, at the same places along the coast, the low-lying areas turn into lagoons and marshy lands.
(3) Places located on the opposite meridian to the place experiencing high or low tide will also experience high or low tide respectively.
Ans:
(i) When a particular place (located on a particular meridian) faces the moon, the moon’s gravitational force exceeds the earth’s centrifugal force as the water is pulled towards the moon.
(ii) At the same time, at an antipodal location (on the opposite meridian), the centrifugal force of the earth exceeds the gravitational force of the moon, and the water is pulled in the opposite direction from the moon.
(iii) The water required for the high tide moves in from the places on the two opposite meridians that are at right angles to those having high tides causing low tides at those places located on the opposite meridian to experience high or low tides, respectively.
3. Answer in brief.
(1) If there is a high tide at 7 am, find the timings of the next high and low tides on the same day at a given place.
Ans: If there is a high tide at 7 a.m., the timing of the next day’s low tide and high tide will be approximately 1.12 p.m. and 7.25 p.m., respectively.
(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.
Ans: There will be high tide too at 107⁰ W meridian, which is exactly opposite the 73⁰ E meridian. The reason behind this is that a place on earth located at the opposite point of the place experiencing high or low tide also experiences high or low tides at the same time.
(3) Explain the reasons for the generation of waves.
Ans: The main reason for the generation of waves is wind. Sometimes sea waves are generated due to earthquakes or volcano eruptions below the floor of the sea.
4. In what way will the following depend on the tides?
(1) Swimming
Ans: While entering the sea, one must have adequate information about the tides. Swimming in deep water during tides can lead to serious accidents. During high tides, it is always safe to swim in shallow water near the coast.
(2) Steering a ship
Ans: Ships can move up to the ports during the high tides. Therefore, one must steer the ship by taking into account the timing of the tides.
(3) Fishing
Ans: With the high tide, fish move into the creeks. Therefore, fishing is largely carried out at high tide.
(4) Salt pans
Ans: During the high tides, seawater can be stored in the saltpans; from this water, salt can be obtained.
(5) Going to coastal areas for trips
Ans: While going to coastal areas for trips, one must have details about the timing of the tides, the structure, and the slope of the coast. Rocky areas stream near the coast before entering the seawater for enjoyment.
5. Observe fig. 3.8 of neap tide and answer the following questions.
(1) Which phase of the moon does it show?
Ans: It shows a phase of the quarter moon.
(2) What are the relative positions of the moon, the sun and the earth?
Ans: The relative positions of the moon, the sun and the earth are such that the sun and the earth make an angle of 90⁰.
(3) What effect will it have on the tides?
Ans: If such is the position of the moon, the sun and the earth, it will lead to neap tides.
6. Differentiate between
(1) High tide and low tide.
Ans:
| High Tide | Low Tide |
|---|---|
(i) A rise in the sea level is called the high tide. | (i) A fall in the sea level is called a low tide. |
(ii) At high tide, sea water is very close to the coast. | (ii) At low tide, sea water is far away from the coast. |
(2) Spring tide and neap tide.
Ans:
| Spring Tide | Neap Tide |
|---|---|
(i) Spring tide occurs on a full moon day and a new moon day. | (i) Neap tide occurs on a quarter moon day. |
(ii) During spring tides, the total pull and recede of sea water is much higher than the average pull and recede. | (ii) During neap tides, the total pull and recede of sea water is much lower than the average full pull and recede. |
7. Describe the positive and negative effects of tides.
Ans: The effects of tides are as follows:
Positive effects:
(i) With the high tides, fish move into the creeks, which helps fishing activities.
(ii) The tides clear the waste, and hence the coasts become clean.
(iii) Ports do not get filled with sediment.
(iv) Ships can move up to the ports during high tide.
(v) During high tide, seawater can be stored in saltpans from which salt can be obtained.
(vi) The tidal force can be used to generate electricity.
(vii) The tides help maintain the mangroves and coastal biodiversity.
Negative effects:
A lack of understanding of the timing of high and low tides may cause accidents for swimmers entering the sea.