Meaning of Tides and the Forces Behind Them
If you stay at the seashore for some time the water sometimes comes near the shore and sometimes goes far. This is high tide and low tide.
Tides are the daily and regular movements of sea water. A cycle of high tide and low tide completes after every 12 hours 25 minutes.
Key idea 1 : What are tides? — Near one coast the sea water is sometimes very close (high tide) and at other times far away (low tide). Barring a few exceptions all coasts on earth experience tides. The event looks simple but is directly related to the sun, the moon and the earth and the gravitational and centrifugal forces between them.
Key idea 2 : Centrifugal force — If chalk is kept on a notebook and the notebook moved sharply, water in a glass whirled, or a bunch of keys swung around a finger, objects get thrown outward. This is centrifugal force: a force moving away from the centre. It acts opposite to the gravitational force. Leaning outward in a fair’s Ferris wheel is also its effect.
Key idea 3 : Two forces on earth — Due to the earth’s rotation a force moving away from the centre is generated. One may think objects would be thrown into space, but gravitational force acts towards the earth’s centre at the same time and is many times greater, so objects stay in place. Causes of tides: gravitational pull of the moon and sun, revolution of the earth and the indirect revolution of the moon, and centrifugal force.
- Easy : One tide cycle lasts? — 12 h 25 min.
- Medium : Centrifugal force acts? — Away from centre.
- Exam-type : Leaning out in the Ferris wheel is due to? — Centrifugal force.
In the figure: Forces behind tides.
Tide = moon + centrifugal force.
Centrifugal force is not greater than gravity; it is smaller, so objects stay on earth.
Memory clue: Gravity = inward; centrifugal = outward.
Spot the difference
| Force | Direction | Example |
|---|---|---|
| Gravitational | Towards centre | Objects stay on ground |
| Centrifugal | Away from centre | Outward pull in a Ferris wheel |
Practice questions
- 1What are tides?
- 2In how much time does one tide cycle complete?
- 3What is centrifugal force?
- 4State the difference between centrifugal and gravitational force.
- 5Why are objects not thrown into space?
- 6State the factors responsible for tides.
- 7Write two examples showing centrifugal force.
- 8Against which force does a rocket work when going to space?
Show answers (try them yourself first)
- 1Daily and regular movements of sea water.
- 212 hours 25 minutes.
- 3A force moving away from the centre.
- 4Gravity inward, centrifugal outward; gravity is greater.
- 5Because gravitational force is many times greater.
- 6Moon-sun gravity, revolution and centrifugal force.
- 7A swung key bunch; water in a mixer.
- 8Against the earth’s gravitational force.
Spring Tide and Neap Tide
In tug of war when two pull the same way the force adds up; pulling across weakens it. The sun’s and moon’s pulls behave likewise.
On new and full moon days the sun’s and moon’s pulls act in the same direction giving spring tide. On quarter days they act at right angles giving neap tide.
Key idea 1 : Why the moon is effective — The moon is closer to the earth than the sun so its gravitational pull is more effective. A place opposite to one having high or low tide has the same at the same time; this is the effect of centrifugal force. If there is high tide at 0° meridian there is also at 180°, and meridians at right angles to them have low tide.
Key idea 2 : Spring tide — On new moon and full moon days the gravitational pull of the sun and the moon act in the same direction. The total pull increases and the tide is much higher than the average high tide: spring tide. As the bulge is greater the water recedes much more at low tide. The water level in spring tide is higher than average high tide and lower than average low tide.
Key idea 3 : Neap tide — While revolving around the earth the moon makes a right angle with the earth and the sun twice a month, on the first and third quarters. The sun’s and moon’s forces act at right angles and are not complementary. So the water level rises less at high tide and falls less at low tide: neap tide, a little lower than average high tide and a little higher than average low tide.
- Easy : Spring tide on which days? — New and full moon.
- Medium : Neap tide on which days? — First and third quarter.
- Exam-type : High tide at 0° meridian; at 180°? — High tide.
In the figure: Two kinds of tides.
Spring = aligned = highest; neap = right angle = smaller.
Neap tide is not “no tide”; it is lower in height.
Memory clue: Aligned → spring; right angle → neap.
Spot the difference
| Point | Spring tide | Neap tide |
|---|---|---|
| Day | New, full moon | First, third quarter |
| Sun-moon pull | Same direction | At right angle |
| High tide | Much higher | Lower |
Practice questions
- 1Why is the moon’s pull more effective than the sun’s?
- 2When does a spring tide occur?
- 3When does a neap tide occur?
- 4State the difference between the two types of tides.
- 5Why does the water recede more at spring low tide?
- 6If high tide is at 0° and 180°, where is low tide?
- 7How is the level of a neap tide?
- 8How many times in a month does a spring tide occur?
Show answers (try them yourself first)
- 1Because the moon is closer to the earth.
- 2On new and full moon.
- 3On first and third quarter.
- 4Spring higher; neap lower.
- 5Because the bulge is greater.
- 6At 90° and 270° meridians.
- 7Slightly lower than average high tide.
- 8Twice (new moon, full moon).
Timing and Effects of Tides
Checking the tide time before visiting the beach avoids danger. The tide time shifts a little later daily.
The time between two high tides is about 12 hours 25 minutes. Tides affect fishing, ports, salt pans and electricity generation.
Key idea 1 : Why timings change daily — When any point of the earth comes opposite to the moon there is high tide. The earth takes 24 hours for 360°, i.e. 4 minutes per degree. While the earth rotates the moon also moves ahead by about 6°15′. So the point takes about 25 minutes more to come before the moon again. So generally there are two high and two low tides daily and the gap between two high tides is 12 hours 25 minutes.
Key idea 2 : Benefits of tides — With high tide fish move into creeks, helping fishing. Tides clear waste and keep coasts clean. Ports do not fill with sediment and ships can move up to ports at high tide. Sea water can be stored in salt pans to get salt. Tidal force can generate electricity. Tides help maintain mangroves and coastal biodiversity.
Key idea 3 : Caution and tidal range — Not knowing the tide timings can cause accidents to swimmers; before entering the sea learn from local people about the coast’s structure, slope, rocky areas and nearby streams. The difference in water level between high and low tide is the tidal range. In open seas it is about 30 cm; along the Peninsular Indian coast mostly 100 to 150 cm. The highest tidal range in the world is at the Bay of Fundy, about 1600 cm, and in India at the Gulf of Khambhat, about 1100 cm.
- Easy : Time for the earth to turn 1°? — 4 minutes.
- Medium : Tidal range in open sea? — About 30 cm.
- Exam-type : Highest tidal range in India? — Gulf of Khambhat.
In the figure: Tide times shift later daily.
As per the textbook the world’s highest tidal range is at the Bay of Fundy (about 1600 cm) and in India at the Gulf of Khambhat (about 1100 cm). (as per the textbook)
12 h 25 min = half of 24 h 50 min.
High tide does not come at the same time daily; it shifts about 50 minutes later each day.
Memory clue: 1° = 4 min; 6°15′ = 25 min.
Spot the difference
| Effect | Example | Benefit/risk |
|---|---|---|
| Fishing | Fish enter creeks | Benefit |
| Port | No silting | Benefit |
| Swimming | Not knowing timings | Risk |
Practice questions
- 1What is the gap between two high tides?
- 2Why do tide timings change daily?
- 3Write four benefits of tides.
- 4What is tidal range?
- 5Where is the highest tidal range in India?
- 6What care should be taken before entering the sea?
- 7How is tidal water useful for salt pans?
- 8How long will the earth take to turn 6°15′?
Show answers (try them yourself first)
- 112 hours 25 minutes.
- 2The moon also moves ahead so it takes 25 min more.
- 3Fishing, clean coasts, ports, salt, electricity.
- 4Difference between high and low tide levels.
- 5Gulf of Khambhat.
- 6Know tide timings and local coast details.
- 7Water stored at high tide yields salt.
- 8About 25 min (6°15′ × 4 min).
Waves and Tsunami
Blow on hot tea and ripples form on the surface. When winds blow on the sea similar waves form.
Ripples raised on the sea surface by wind are waves. Highly destructive waves caused by an earthquake or volcanic eruption below the sea floor are tsunamis.
Key idea 1 : Waves — Sea water is pushed by wind and ripples form on the surface; these are waves. Waves move water up and down or slightly forward and backward. They bring their energy to the coast and break in shallow water. Strong wind in one direction creates large waves.
Key idea 2 : Structure and motion — The raised part of a wave is the crest and the depressed part the trough. The vertical distance between a crest and the next trough is the amplitude and the distance between two crests is the wavelength. All depend on wind speed. A floating object does not come to the shore with the wave; it moves up and down at the same place. So the water in a wave does not move forward; energy does.
Key idea 3 : Tsunami — Though wind is the main reason, sometimes earthquakes or volcanic eruptions below the sea floor also generate waves. In shallow water near the coast they become very high and cause huge loss: tsunamis. In 2004 tsunami waves caused by an earthquake near Indonesia’s Sumatra island devastated India’s east coast and Sri Lanka. If an earthquake occurs in the ocean coastal areas face a risk; then move away from the coast or to higher ground.
- Easy : Main cause of waves? — Wind.
- Medium : What moves forward in a wave? — Energy, not water.
- Exam-type : The 2004 tsunami was due to an earthquake near? — Sumatra.
In the figure: Tsunami from a seabed earthquake.
In 2004 tsunami waves caused by an earthquake near Indonesia’s Sumatra island devastated India’s east coast and Sri Lanka. (as per the textbook)
Crest = up; trough = down; energy travels, not water.
Water does not travel to the shore with the wave; it moves up and down in place.
Memory clue: Wave = energy; tsunami = earthquake wave.
Spot the difference
| Point | Waves | Tsunami |
|---|---|---|
| Cause | Wind | Earthquake/volcano |
| Effect | Coastal erosion | Huge destruction |
Practice questions
- 1What causes waves?
- 2What are crest and trough?
- 3State amplitude and wavelength.
- 4Does water move forward in a wave? Explain.
- 5What is a tsunami?
- 6State the effect of the 2004 tsunami.
- 7What precaution after an earthquake in the sea?
- 8What changes do waves cause along a coast?
Show answers (try them yourself first)
- 1The force of wind.
- 2Raised part crest; depressed trough.
- 3Crest-trough vertical gap amplitude; crest-crest gap wavelength.
- 4No; energy goes forward, water moves up and down.
- 5A big wave from a seabed quake/volcano.
- 6Great damage to India’s east coast and Sri Lanka.
- 7Go away from the coast or to higher ground.
- 8Erosion of headlands and beaches formed in bays.