Tides | Class 7 | Geography | Chapter 3 | Maharashtra State Board
Welcome to this informative video on tides and waves. In this video, we will explore the fascinating world of tides and waves and their causes. We will begin by discussing the centrifugal and gravitational forces of the sun, moon, and the earth, which are responsible for the tides. We will explain how these forces interact with each other and affect the ocean's water levels. Next, we will delve into the different types of tides - spring tides and neap tides. We will explain how they are formed, their effects, and the factors that influence their intensity. We will also calculate how the timings of tides change daily and how they vary according to the location of the observer. Moving on, we will discuss waves and their characteristics. We will explore the different types of waves, including ocean waves, tsunamis, and tidal waves. We will discuss their structures, velocities, and how they are affected by various factors such as wind speed and direction. Throughout the video, we will use visual aids and diagrams to help you better understand the complex concepts of tides and waves. By the end of this video, you will have a better understanding of the fascinating world of tides and waves and how they impact our lives. So, join us on this journey of exploration and learn more about these natural wonders!
Questions & Answers
Q.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 volcones. |
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. |
Answer:
Group A | Answer | Answer |
Waves | Wind | These are also generated due to earthquakes and volconoes. |
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. |
Q.2. Give geographical reasons.
1. Tides are influenced more by the moon than the sun.
Ans. The gravitational pull of the moon and the sun as well as that of the earth is one of the factor responsible for occurrence of tides. But compared to the sun, the moon is closer to the earth. Due to this the gravitational force of the moon becomes more effective than that of the sun. Therefore, tides are influenced more by the moon than the sun.
2. At some places along the coast, the low lying areas turn into lagoons or marshy lands.
Ans.
Generally the areas along the sea coast are low lying and get flooded easily by the sea water during high tide.
This leads to accumulation of sea water over a long period of time leading to formation of swamps and marshes.
The tides help in maintaining the mangroves and the coastal biodiversity here.
Certain areas near the sea coast being low lying and waters being shallow also lead to sediment deposition by sea waves leading to formation of lagoons.
In this way, at some places along the coast, the low lying areas turn into lagoons or marshy lands.
3. Place located on the opposite meridian experiencing high or low tide will also experience high or low tide respectively.
Ans.
The gravitational force of moon, the sun and the earth and the centrifugal force generated due to the rotation of the earth are two major factors responsible for the occurrence of tides.
When a place (meridian) faces the moon the gravitational force of the moon exceeds the centrifugal force of the earth leading to high tide here as the water is pulled towards the moon.
At the same time the place on the earth located at the opposite meridian to the place experiencing high tide, the centrifugal force of the earth exceeds the gravitational force of the moon. Thus, the water is pulled in the direction away from the moon leading to high tide.
The water required for the high tide moves in from places that are at right angles to those having high tides causing a low tide at those places.
Thus, place located on the opposite meridian to the place experiencing high or low tide will also experience high or low tide respectively.
Q.3. Answer in brief.
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.
Ans. There is high tide and low tide twice a day i.e. in 24 hours. One cycle of high tide and low tide is completed after every 12 hours and 25 minutes. If there is a high tide at 7 am, the next low tide will be at 1.12 pm (after 6 hrs. and 12 min.) and the next high tide will be at 7.25 pm (after 12 hrs. and 25 min.).
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 a high tide too? State with reasons.
Ans. The other meridian which will experience high tide will be 107°W (180°-73°), because, it is exactly 180° opposite from 73°E.
3. Explain the reasons for the generation of waves.
Ans. Large or small waves are formed continuously at the surface of the sea. The main reason of wave generation is the force of the wind i.e. water appears to be moving. Generation of waves is a natural and regular phenomenon. But at times waves get generated due to earthquake or volcanic eruptions occurring below the floor of the sea. These waves causes huge loss of property and life.
Q.4. In what way will be following depend on the tides?
1. Swimming
Ans. A lack of an understanding of the timings of high and low tides may cause accidents to swimmers entering the sea.
2. Steering a ship
Ans. Ships can move up to the ports during high tide.
3. Fishing
Ans. With the high tide, fish moves into the creeks and this helps the fishing activity.
4. Salt Pans
Ans. During high tide, seawater can be stored in salt pans from which salt can be obtained.
5. Going to coastal areas for trips
Ans. Plan for going to trips after full moon or new moon day. Also one must have details about the timings of the tides for better enjoyment.
Q.5. Observe figure 3.8 of neap tide and answer the following questions.
1. Which phase of the moon does it show?
Ans. The diagram shows the phase of the quarter moon.
2. What are the relative positions of the moon, the sun and the earth?
Ans. The moon, the earth and the sun make an angle of 90° (right angle).
3. What effect will it have on the tides?
Ans. It will lead to neap tide. Due to such conditions during high tide the water level will rise less than usual while during low tide water level will fall less than usual.
Q.6. Differentiate between.
1. High tide and low tide.
| High tide | Low tide |
1. | The rise in the level of sea water is caused by the combined effect of centrifugal and gravitational force of the moon and the sun. | The fall in the level of sea water is caused by the combined effect of centrifugal and gravitational force of the moon and the sun. |
2. | At the time of high tide, sea water is very close to the coast. | At the time of low tide, sea water is far away from the coast. |
2. Spring tide and neap tide.
| Spring tide | Neap tide |
1. | It occurs on the new moon day and the full moon day. | It occurs on the first and the third quarted days. |
2. | The gravitational pull of the moon and the sun as well as that of the earth are in a straight line and act in the same direction. | The gravitational pull of the moon and the sun are at right angle. |
3. | Spring tides are a little higher than the average high tides and a little lower than the average low tides. | Neap tides are a little lower than the average high tides and little higher than the average low tides. |
Q.7. Describe the positive and negative effects of tides.
Ans. The positive effects of tides are as follows:
The tides clear the waste and hence the coasts become clean.
Ports do not get filled with sediments .
Ships can move up to the ports during high tide.
During high tide, sea water can be stored in salt pans from which salt can be obtained.
The tidal force can be used to generate electricity.
With the high tide, fish move into the creeks and this helps fishing activity.
The tides help in maintaining the mangroves and the coastal biodiversity.
The negative effects of tides are as follows:
A lack of an understanding of the timings of high and low tides may cause accidents to swimmers entering the sea.
E X T R A
Q.1. Differentiate between.
1. Centrifugal force and Gravitational force.
| Centrifugal force | Gravitational force |
1. | Due to rotation, the earth gets a type of power of force. This force works away from the centre. It is called centrifugal force. | Gravitational force is working towards the centre of the earth. This force is many times greater than the centrifugal force. |
2. | Centrifugal force works away from the centre. | Gravitational force works towards the centre. |
3. | Due to centrifugal force an object on the earth would be thrown into the space. | Due to gravitational force an object on the surface of the earth remains at the place where it exists. |
Q.2. Write short notes.
1. Spring tide.
Ans.
On new moon and full moon days, the gravitational pull of the sun and the moon act in the same direction due to which the total pull increases.
Hence, the tide on these days is much higher than the average high tide.
This is known as spring tide.
2. Neap tide.
Ans.
While revolving around the earth, the moon makes a right angle with respect to the earth and the sun, twice a month.
This position occurs on the first and third quarter of each month. On both these days the forces of both the sun and the moon operate at right angles on the earth.
At the places where the sun causes high tide, the gravitational pull of the moon which is at right angles also acts on the water.
Due to such conditions the water level rise is less than usual at the time of high tide. Similarly, fall in water level is less than usual at the time of low tide.
Such tides are called neap tides.
3. Waves.
Ans. The sea water gets pushed by the wind and ripples are generated on the water surface. These are called waves. The sea water moves up and down or slightly forward and backward due to the waves. The waves bring the energy contained in them to the coast. Large and small waves are formed continuously at the surface of the sea. Generation of waves is a natural and regularly occurring phenomenon.
4. Structure of the waves.
Ans.
The sea water gets pushed up and down because of the wind. The raised up portion of the wave is called crest and the depressed one is called trough.
The vertical distance between a crest and the following trough is called the amplitude of the wave whereas the distance between two successive crests or troughs is called wave length.
The wave length, its amplitude and its velocity depend on the velocity of the wind.
Q.3. Fill in the blanks choosing the correct options from the brackets.
Due to rotation, the force that works away from the centre of the earth is the centrifugal force. (gravitational, rotational, centrifugal)
Neap tide occurs on the days of the first and third quarter of each month. (first, second, third)
The tidal range in open seas is 30 cms. (20, 30, 40)
The highest tidal range in the world is observed at Bay of Fundy. (Bay of Fundy, Bay of Biscay, Bay of Bengal)
The main reason for wave generation is wind. (wind, gravitational force, centrifugal force)
The raised up portion of a wave is called a crest. (trough, wave length, crest)
The depressed portion of a wave is called a trough. (trough, wave length, crest)
The vertical distance between a crest and a trough is called the amplitude. (wave length, amplitude, tide)
The distance between a crest or trough is called wave length. (amplitude, wave length, tide)
Tall sea waves caused by earthquakes below the floor of the sea are called tsunamis. (cyclones, tsunamis, eruption)
Q.4. Define the following.
1. Tide.
Ans. The alternate rising and falling of the sea water after a specific period is called tide.
2. Centrifugal force.
Ans. Due to the earth’s rotation, the earth gets a type of power or force. This force works away from the centre. It is called the centrifugal force.
3. Tidal range.
Ans. The difference in the water level of the high tide and low tide is called tidal range.
4. Waves.
Ans. The sea water gets pushed by the wind and so ripples are generated on the water surface. These are called waves.
5. Amplitude of the wave.
Ans. The vertical distance between a crest and the following trough is called the amplitude of the wave.
6. Wave length.
Ans. The wave length is the distance between two successive crests or troughs.
Q.5. Give geographical reasons.
1. Any object on the surface of the earth remains at the place where it exists.
Ans. Due to the earths rotation, it gets a type of power or force. The force acting in the centre is centrifugal force. Due to this force any object on the earth would be thrown into the space. But the gravitational force is working towards the centre of the earth at the same time. This force is greater than the centrifugal force. Hence, any object on the surface of earth remains at the place where it exists.
2. On new moon and full moon days the tides are higher than average.
Ans. On new moon and full moon days, the gravitational pull of the sun and the moon act in the same direction. Due to this, the total pull increases. Hence, the tide on these days is much higher than the average high tide.
3. Neap tides are a little lower than average high tides.
Ans. While revolving around the earth, the moon makes a right angle with respect to the earth and the sun. This position occurs on the first and the third quarter of each month. On both these days, the forces of both the sun and the moon operate at right angles on the earth. The attraction of the sun and the moon are not complementary but at right angles to each other. Due to this, the water level rise is less than usual at the time of high tide.
Q.6. Answer in brief.
1. Which factors are responsible for the occurrence of tides?
Ans. The following factors are responsible for the occurrence of tides:
The gravitational pull of the moon and the sun as well as that of the earth.
Revolution of the earth around the sun and the indirect revolution of the moon around the sun.
Centrifugal force generated due to the rotation of the earth.
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