[{"data":1,"prerenderedAt":1147},["ShallowReactive",2],{"questions:tides":3},{"bank":4,"contentHash":1136,"dependencyHashes":1137,"releaseId":1146},{"schemaVersion":5,"conceptId":6,"revision":5,"title":7,"intro":8,"sections":9,"questions":42,"sourceIds":1124,"reviewStatus":1132,"authoring":1133},1,"tides","Tides question bank","Seventy-plus questions covering the whole of Tides: what a tide is, why the Moon raises two bulges, the 24 h 50 min tidal day, spring and neap tides, why range varies so much by place, tide types and tables, tides and people, and tidal friction across deep time. Work through a section at a time, or dip in wherever you like — every question has a full worked solution.",[10,14,18,22,26,30,34,38],{"id":11,"title":12,"description":13},"tide-basics","What a tide is","Tide vs wave vs tsunami, high and low water, tidal range, and basic vocabulary.",{"id":15,"title":16,"description":17},"bulges","The two bulges","Why the Moon's differential pull raises two tidal bulges, and the misconceptions to avoid.",{"id":19,"title":20,"description":21},"tidal-day","The tidal day","The 24 h 50 min lunar day, the 50-minutes-later rule, and tide-time arithmetic.",{"id":23,"title":24,"description":25},"spring-neap","Spring and neap tides","The Sun's contribution, alignment at new and full moon, and the fortnightly rhythm.",{"id":27,"title":28,"description":29},"range-and-place","Why range varies by place","Funnelling, shallowing, resonance, and real Indian and world examples of tidal range.",{"id":31,"title":32,"description":33},"tide-types","Tide types and tables","Semidiurnal, diurnal and mixed tides, the rule of twelfths, and how tide tables are made.",{"id":35,"title":36,"description":37},"tides-and-people","Tides and people","Fishing, ports, salt pans, tidal power, storm surges, tidal bores and coastal safety.",{"id":39,"title":40,"description":41},"deep-time","Deep time and beyond","Tidal friction, the receding Moon, fossil evidence for ancient day length, and tidal heating on other moons.",[43,68,86,100,111,130,145,158,177,195,213,231,242,261,272,291,309,328,340,352,363,374,392,410,421,439,450,461,479,498,516,535,547,558,569,580,598,617,632,642,661,679,690,708,727,746,765,775,787,798,817,828,838,857,868,887,906,924,936,945,963,974,992,1010,1029,1040,1051,1060,1070,1088,1106],{"id":44,"section":11,"level":45,"prompt":46,"check":47,"hints":63,"solution":65,"skills":66},"tides.q001","foundation","What is a tide?",{"kind":48,"options":49,"correct":62},"choice",[50,53,56,59],{"id":51,"label":52},"a","A large wave caused by wind",{"id":54,"label":55},"b","The regular rise and fall of the sea, usually twice a day",{"id":57,"label":58},"c","A current that always flows the same way",{"id":60,"label":61},"d","A wave caused by an earthquake",[54],[64],"Think about what changes over about six hours, not a few seconds.","A **tide** is the regular rise and fall of the whole sea, usually **twice a day**, caused mainly by the Moon's gravity. Waves are wind-driven; tsunamis are made by earthquakes or landslides.",[67],"definitions",{"id":69,"section":11,"level":45,"prompt":70,"check":71,"hints":82,"solution":84,"skills":85},"tides.q002","What is the difference between a wave and a tide?",{"kind":48,"options":72,"correct":81},[73,75,77,79],{"id":51,"label":74},"There is no difference",{"id":54,"label":76},"A wave moves water up and down in one place for seconds; a tide moves the whole sea level over hours",{"id":57,"label":78},"A wave is caused by the Moon; a tide is caused by wind",{"id":60,"label":80},"A tide only happens at night",[54],[83],"Which one is caused by wind, and which by the Moon?","**Waves** are quick, wind-made ripples lasting seconds. A **tide** is the slow rise and fall of the entire sea level, taking about six hours each way.",[67],{"id":87,"section":11,"level":45,"prompt":88,"check":89,"hints":94,"solution":96,"skills":97},"tides.q003","High water is 5.2 m and low water is 1.4 m. What is the tidal range?",{"kind":90,"answer":91,"tolerance":92,"unit":93},"number",3.8,0.05,"m",[95],"Range = high water − low water.","Range = 5.2 − 1.4 = **3.8 m**.",[98,99],"arithmetic","tidal range",{"id":101,"section":11,"level":102,"prompt":103,"check":104,"hints":106,"solution":108,"skills":109},"tides.q004","core","A tide board reads 6.1 m at high water and 0.9 m at low water. What is the tidal range, and is this closer to Kochi's typical range or Mumbai's?",{"kind":90,"answer":105,"tolerance":92,"unit":93},5.2,[107],"Subtract to get the range, then compare with the typical figures from Discover (Kochi ≈1 m, Mumbai ≈4.4 m).","Range = 6.1 − 0.9 = **5.2 m**, bigger than Mumbai's typical 4.4 m spring range and far bigger than Kochi's typical 1 m — closer to a Hooghly-estuary-sized tide.",[98,99,110],"comparison",{"id":112,"section":11,"level":102,"prompt":113,"check":114,"hints":125,"solution":127,"skills":128},"tides.q005","Why is a tsunami not the same thing as a tide, even though it is sometimes wrongly called a 'tidal wave'?",{"kind":48,"options":115,"correct":124},[116,118,120,122],{"id":51,"label":117},"A tsunami is bigger, that is the only difference",{"id":54,"label":119},"A tsunami is made by an earthquake or landslide, arrives without a fixed timetable, and is not caused by the Moon or Sun",{"id":57,"label":121},"A tsunami only happens at low tide",{"id":60,"label":123},"There is no real difference",[54],[126],"Think about what causes each one, and whether either can be predicted years ahead.","A tide is caused by the Moon and Sun and can be predicted years ahead. A **tsunami** is caused by an earthquake, volcano or landslide moving the sea bed, and arrives with at best a few hours' warning.",[129],"misconceptions",{"id":131,"section":11,"level":132,"prompt":133,"check":134,"hints":140,"solution":142,"skills":143},"tides.q006","stretch","A harbour's tide gauge shows the water level rising steadily from 0.5 m to 4.9 m over six hours, then falling back to 0.6 m over the next six hours. A friend says 'that's a wave.' Explain what is wrong with calling it a wave, using two features of the data.",{"kind":135,"accept":136},"text",[137,138,139],"it takes hours not seconds and it is the whole water level not a ripple","it changes over hours and affects the whole sea level, not just the surface","the timescale is hours not seconds and the whole level rises, unlike a wave",[141],"Waves change in seconds and only ripple the surface; think about both the timescale and what is actually moving.","Two clues: it takes **hours**, not seconds, to rise and fall (waves complete a cycle in a few seconds), and the **whole water level** changes together, not just a ripple on the surface. Both point to a tide.",[144,129],"reasoning",{"id":146,"section":11,"level":147,"prompt":148,"check":149,"hints":153,"solution":155,"skills":156},"tides.q007","challenge","A coastal town experiences three separate events in one week: (1) a gentle rise and fall of the sea over 12 hours, repeated twice a day; (2) choppy 3-second ripples during a windy afternoon; (3) a single, unpredicted wall of water after a distant earthquake. Match each event to its correct name and give one piece of evidence for each match.",{"kind":135,"accept":150},[151,152],"1 tide 2 wave 3 tsunami","tide, wave, tsunami",[154],"Match the timescale and cause of each event to the three categories from this layer.","(1) is a **tide**: a slow, twice-daily rise and fall over hours. (2) is a **wave**: fast, wind-made ripples lasting seconds. (3) is a **tsunami**: a single, unpredicted event triggered by an earthquake, arriving without a tide table's schedule.",[144,157],"classification",{"id":159,"section":15,"level":45,"prompt":160,"check":161,"hints":172,"solution":174,"skills":175},"tides.q008","Why does the Moon's gravity make the ocean move at all, even though the pull is very weak?",{"kind":48,"options":162,"correct":171},[163,165,167,169],{"id":51,"label":164},"The Moon is magnetic",{"id":54,"label":166},"Water is free to flow, and the pull acts on the whole ocean for hours at a time",{"id":57,"label":168},"The Moon's light heats the water",{"id":60,"label":170},"The pull is actually very strong, stronger than Earth's own gravity",[54],[173],"Rock cannot flow easily; water can.","The pull is tiny, but water can **flow**, and the same gentle push acts on an entire ocean continuously for hours, so the tiny nudges add up to a large effect.",[176],"mechanism",{"id":178,"section":15,"level":45,"prompt":179,"check":180,"hints":191,"solution":193,"skills":194},"tides.q009","How many tidal bulges does Earth's ocean have?",{"kind":48,"options":181,"correct":190},[182,184,186,188],{"id":51,"label":183},"One",{"id":54,"label":185},"Two",{"id":57,"label":187},"Four",{"id":60,"label":189},"None",[54],[192],"One faces the Moon; what about the opposite side?","**Two**: one on the side facing the Moon, and one on the side facing directly away.",[176],{"id":196,"section":15,"level":102,"prompt":197,"check":198,"hints":209,"solution":211,"skills":212},"tides.q010","What causes the far-side tidal bulge?",{"kind":48,"options":199,"correct":208},[200,202,204,206],{"id":51,"label":201},"Earth's spin flings the water outward",{"id":54,"label":203},"The Moon pulls the solid Earth more than it pulls the far ocean, so Earth is pulled out from under that water",{"id":57,"label":205},"The Sun pushes the water away from the Moon",{"id":60,"label":207},"The far ocean is closer to the Moon than the near ocean",[54],[210],"Rank the pull on the near ocean, the solid Earth, and the far ocean from strongest to weakest.","The Moon pulls the near ocean hardest, the solid Earth a little less, and the far ocean least. The solid Earth is pulled out from under the far ocean, leaving it behind — a heap of water, not water flung off by spin.",[129,176],{"id":214,"section":15,"level":102,"prompt":215,"check":216,"hints":227,"solution":229,"skills":230},"tides.q011","If the Moon pulled on every part of Earth with exactly the same force, what would happen to the tides?",{"kind":48,"options":217,"correct":226},[218,220,222,224],{"id":51,"label":219},"They would be twice as big",{"id":54,"label":221},"There would be no tides at all",{"id":57,"label":223},"There would be only one bulge",{"id":60,"label":225},"Nothing would change",[54],[228],"A force that is the same everywhere moves everything together.","**No tides at all.** An equal pull everywhere accelerates Earth and its ocean together, so nothing moves relative to anything else. Tides come from the *difference* in pull, not its overall strength.",[176,144],{"id":232,"section":15,"level":132,"prompt":233,"check":234,"hints":238,"solution":240,"skills":241},"tides.q012","Explain, in two or three sentences, why 'the far bulge is flung off by Earth's spin' is a misconception, using one piece of evidence that rules it out.",{"kind":135,"accept":235},[236,237],"the far bulge always points opposite the moon and follows it, spin cannot explain that; also the moon itself has tides and spins slowly","it always stays opposite the moon and the moon has tides too despite spinning slowly",[239],"Does the far bulge track the Moon, or stay fixed relative to Earth's spin? Does a slowly spinning body ever get tides?","If spin flung the bulge off, it would not care where the Moon was — but the far bulge always points exactly opposite the Moon and follows it around. Also, the Moon itself has tides raised on it by Earth despite barely spinning at all, so spin cannot be the cause.",[144,129],{"id":243,"section":15,"level":132,"prompt":244,"check":245,"hints":256,"solution":258,"skills":259},"tides.q013","The near side of Earth is about 1.7% closer to the Moon than Earth's centre. Using the rule that gravity varies as 1 ÷ distance², is the pull on the near side more than 1.7% stronger, less than 1.7% stronger, or exactly 1.7% stronger than at the centre?",{"kind":48,"options":246,"correct":255},[247,249,251,253],{"id":51,"label":248},"Exactly 1.7% stronger",{"id":54,"label":250},"More than 1.7% stronger, because the effect is squared",{"id":57,"label":252},"Less than 1.7% stronger",{"id":60,"label":254},"There is no way to tell",[54],[257],"(1.017)² is bigger than 1.017.","**More than 1.7%.** Since pull goes as 1 ÷ distance², a 1.7% smaller distance gives (1.017)² ≈ 1.034, about 3.4% more pull — squaring a number bigger than 1 makes it even bigger.",[144,260],"percentages",{"id":262,"section":15,"level":147,"prompt":263,"check":264,"hints":268,"solution":270,"skills":271},"tides.q014","A student argues: 'if the Moon's pull on the near ocean is only about 3.4% stronger than on the solid Earth, that tiny difference can't possibly move an entire ocean.' Give the strongest counter-argument from this topic.",{"kind":135,"accept":265},[266,267],"the difference acts on the whole ocean continuously for hours so tiny nudges add up over a huge volume of water","small but constant force over a huge ocean and a long time adds up to a large effect",[269],"Think about how a small force can still do a lot if it acts on something huge for a long time.","A tiny force, if it acts on an **entire ocean** continuously for **hours**, adds up: every bucketful of water in the ocean is nudged the same way at once, and that sideways nudge accumulates into metres of water piling up at a coast, exactly the same reasoning that lets a gentle wind fill a huge sail.",[144],{"id":273,"section":19,"level":45,"prompt":274,"check":275,"hints":286,"solution":288,"skills":289},"tides.q015","About how much later does high tide arrive each day, compared with the day before?",{"kind":48,"options":276,"correct":285},[277,279,281,283],{"id":51,"label":278},"About 10 minutes",{"id":54,"label":280},"About 50 minutes",{"id":57,"label":282},"About 2 hours",{"id":60,"label":284},"It arrives at exactly the same time every day",[54],[287],"The Moon moves along its orbit while Earth spins, so Earth has to turn a little extra.","About **50 minutes** later each day, because the Moon has moved on and Earth must turn a little further to catch up with it.",[290],"tidal day",{"id":292,"section":19,"level":45,"prompt":293,"check":294,"hints":305,"solution":307,"skills":308},"tides.q016","How long is a lunar (tidal) day?",{"kind":48,"options":295,"correct":304},[296,298,300,302],{"id":51,"label":297},"23 hours",{"id":54,"label":299},"24 hours exactly",{"id":57,"label":301},"24 hours 50 minutes",{"id":60,"label":303},"25 hours 30 minutes",[57],[306],"It is a little longer than an ordinary day.","A lunar day is **24 h 50 min** — 50 minutes longer than the ordinary 24-hour solar day.",[290],{"id":310,"section":19,"level":102,"prompt":311,"check":312,"hints":323,"solution":325,"skills":326},"tides.q017","High water is at 04:15 today. About what time will the matching high water be tomorrow?",{"kind":48,"options":313,"correct":322},[314,316,318,320],{"id":51,"label":315},"04:15",{"id":54,"label":317},"About 05:05",{"id":57,"label":319},"About 03:25",{"id":60,"label":321},"About 16:15",[54],[324],"Add about 50 minutes.","**About 05:05.** 04:15 + 50 minutes = 05:05, because tides slip about 50 minutes later each day.",[290,327],"clock arithmetic",{"id":329,"section":19,"level":102,"prompt":330,"check":331,"hints":336,"solution":338,"skills":339},"tides.q018","High water at a port is at 06:20. Using half a lunar day, what time is the next high water?",{"kind":135,"accept":332},[333,334,335],"18:45","6:45 pm","18h45",[337],"Add 12 h 25 min to 06:20.","06:20 + 12 h 25 min = **18:45**. (06:20 + 12 h = 18:20; add the remaining 25 min = 18:45.)",[290,327],{"id":341,"section":19,"level":132,"prompt":342,"check":343,"hints":347,"solution":349,"skills":350},"tides.q019","High water is at 03:00 on Monday. Using the 50-minutes-later rule, what time is the matching high water on Friday (4 days later)?",{"kind":135,"accept":344},[345,346],"06:20","6:20 am",[348],"4 days × 50 minutes = 200 minutes = 3 h 20 min. Add that to 03:00.","4 × 50 = 200 minutes = 3 h 20 min. 03:00 + 3 h 20 min = **06:20**.",[290,351],"multi-step arithmetic",{"id":353,"section":19,"level":132,"prompt":354,"check":355,"hints":359,"solution":361,"skills":362},"tides.q020","Explain why a lunar day is longer than 24 hours, using the fact that the Moon moves along its orbit in the same direction Earth spins.",{"kind":135,"accept":356},[357,358],"earth has to turn a little extra to catch up to where the moon has moved to, taking about 50 more minutes","the moon moves on during the day so earth needs extra time to turn back under it",[360],"Think about what 'catching up' with the Moon means for how far Earth has to turn.","While Earth turns once, the Moon has moved a little further along its own orbit (in the same direction), so a point on Earth is no longer facing the Moon after one full spin — Earth must turn a bit further to catch up, and that extra turning takes about 50 minutes.",[144,290],{"id":364,"section":19,"level":147,"prompt":365,"check":366,"hints":370,"solution":372,"skills":373},"tides.q021","In one synodic month of 29.53 days, a coast faces the Sun 30 times but faces the Moon one time fewer. Use this to show that one lunar day equals 29.53 ÷ 29 days, and give the answer in hours and minutes.",{"kind":135,"accept":367},[301,368,369],"24h50m","24 h 50 min",[371],"29.53 ÷ 28.53, then convert the decimal day to hours and minutes.","29.53 ÷ 28.53 ≈ 1.0351 days × 24 = **24.84 hours ≈ 24 h 50 min**.",[144,351],{"id":375,"section":23,"level":45,"prompt":376,"check":377,"hints":388,"solution":390,"skills":391},"tides.q022","Spring tides happen at which Moon phases?",{"kind":48,"options":378,"correct":387},[379,381,383,385],{"id":51,"label":380},"First and last quarter",{"id":54,"label":382},"New moon and full moon",{"id":57,"label":384},"Only at full moon",{"id":60,"label":386},"Whenever it rains",[54],[389],"Think about when the Sun, Earth and Moon line up.","**New moon and full moon** — both are straight-line alignments of the Sun, Earth and Moon, so the two tides add together.",[23],{"id":393,"section":23,"level":45,"prompt":394,"check":395,"hints":406,"solution":408,"skills":409},"tides.q023","What is a neap tide?",{"kind":48,"options":396,"correct":405},[397,399,401,403],{"id":51,"label":398},"The biggest tide of the month",{"id":54,"label":400},"The smallest tidal range of the fortnight",{"id":57,"label":402},"A tide that only happens in spring",{"id":60,"label":404},"A tide with no low water at all",[54],[407],"It happens at the quarter moons.","A **neap tide** is the smallest tidal range of the fortnight, occurring at the first and last quarter moons, when the Sun's pull partly cancels the Moon's.",[23],{"id":411,"section":23,"level":102,"prompt":412,"check":413,"hints":417,"solution":419,"skills":420},"tides.q024","About what fraction of the Moon's tide-raising effect does the Sun contribute?",{"kind":90,"answer":414,"tolerance":415,"unit":416},46,2,"%",[418],"It is a bit less than half.","About **46%** — big enough to matter hugely (it makes spring and neap tides), but well under half of the Moon's effect.",[23,260],{"id":422,"section":23,"level":102,"prompt":423,"check":424,"hints":435,"solution":437,"skills":438},"tides.q025","Why do both new moon AND full moon produce spring tides, even though the Moon is on opposite sides of Earth each time?",{"kind":48,"options":425,"correct":434},[426,428,430,432],{"id":51,"label":427},"Only new moon actually produces a spring tide",{"id":54,"label":429},"Each body makes two bulges, so a straight-line alignment stacks the bulges together whichever end the Sun is at",{"id":57,"label":431},"The Sun disappears at full moon",{"id":60,"label":433},"It is a coincidence with no real cause",[54],[436],"What matters for a spring tide: which end of the line the Sun is at, or whether it's a straight line at all?","What matters is being in a **straight line**, not which end the Sun sits at. Since the Sun and Moon each raise two bulges, a straight-line alignment stacks the bulges together either way.",[144,23],{"id":440,"section":23,"level":132,"prompt":441,"check":442,"hints":445,"solution":447,"skills":448},"tides.q026","A port's spring tidal range is 4.4 m. Using the Moon-to-Sun ratio of about 2.18 to 1, estimate its neap range.",{"kind":90,"answer":443,"tolerance":444,"unit":93},1.6,0.15,[446],"neap ÷ spring = (Moon − Sun) ÷ (Moon + Sun) = (2.18 − 1) ÷ (2.18 + 1).","neap ÷ spring = (2.18 − 1) ÷ (2.18 + 1) ≈ 0.370. Neap range ≈ 4.4 × 0.370 ≈ **1.6 m**.",[449,351],"ratios",{"id":451,"section":23,"level":132,"prompt":452,"check":453,"hints":457,"solution":459,"skills":460},"tides.q027","About how many days pass between one spring tide and the next?",{"kind":90,"answer":454,"tolerance":455,"unit":456},14.8,0.3,"days",[458],"It is about half a synodic month, or the beat between the Moon's and Sun's semidiurnal tides.","About **14.8 days** — roughly half of the 29.53-day synodic month, since spring tides occur at both new and full moon.",[23,144],{"id":462,"section":23,"level":147,"prompt":463,"check":464,"hints":475,"solution":477,"skills":478},"tides.q028","A friend says 'spring tides happen in spring, and neap tides happen in autumn.' Which correction below is fully accurate?",{"kind":48,"options":465,"correct":474},[466,468,470,472],{"id":51,"label":467},"Spring and neap tides have nothing to do with season; both occur about every two weeks, all year, at new\u002Ffull moon and the quarters",{"id":54,"label":469},"Spring tides do happen in spring, but neap tides can happen in any season",{"id":57,"label":471},"Neap tides do happen in autumn, but spring tides can happen in any season",{"id":60,"label":473},"Both parts are correct as stated",[51],[476],"Think about how often the Moon goes through its phases, and what 'spring' originally meant.","Both parts are wrong: spring and neap tides have **nothing to do with the season**. 'Spring' means water springing up, not the season. Spring tides occur about every 14.8 days, at every new and full moon, all year round, and neap tides occur just as often at the quarter moons.",[144,129],{"id":480,"section":27,"level":45,"prompt":481,"check":482,"hints":493,"solution":495,"skills":496},"tides.q029","Which of these has the world's largest tidal range?",{"kind":48,"options":483,"correct":492},[484,486,488,490],{"id":51,"label":485},"Kochi",{"id":54,"label":487},"The Bay of Fundy",{"id":57,"label":489},"The open mid-Pacific",{"id":60,"label":491},"Chennai",[54],[494],"It is in Canada.","The **Bay of Fundy**, in Canada, has the world's largest tidal range, about 16 m.",[497],"place-and-range",{"id":499,"section":27,"level":45,"prompt":500,"check":501,"hints":512,"solution":514,"skills":515},"tides.q030","What TWO things about a coast's shape help make its tidal range bigger?",{"kind":48,"options":502,"correct":511},[503,505,507,509],{"id":51,"label":504},"Being very deep and very wide",{"id":54,"label":506},"Funnelling (narrowing) and shallowing",{"id":57,"label":508},"Being close to the equator",{"id":60,"label":510},"Having lots of rainfall",[54],[513],"Think of the Gulf of Khambhat's shape.","**Funnelling** (a narrowing coastline) and **shallowing** (a rising sea bed) both squeeze the same volume of water into less room, forcing the level up.",[497],{"id":517,"section":27,"level":102,"prompt":518,"check":519,"hints":530,"solution":532,"skills":533},"tides.q031","Why does the Bay of Fundy have such an enormous tidal range, beyond just being a funnel shape?",{"kind":48,"options":520,"correct":529},[521,523,525,527],{"id":51,"label":522},"It is the deepest bay in the world",{"id":54,"label":524},"Its natural sloshing period is close to the tidal period: resonance",{"id":57,"label":526},"The Moon is closer to Canada",{"id":60,"label":528},"It never has calm weather",[54],[531],"Think of a swing being pushed at just the right moment.","**Resonance.** The bay's natural sloshing rhythm is close to the tide's own beat, so each new tide arrives like a well-timed push on a swing, building an enormous range over time.",[497,534],"resonance",{"id":536,"section":27,"level":102,"prompt":537,"check":538,"hints":541,"solution":543,"skills":544},"tides.q032","Using the Bay of Fundy's average depth of about 75 m and v = √(g × depth), what is the approximate shallow-water wave speed there, in m\u002Fs?",{"kind":90,"answer":539,"tolerance":5,"unit":540},27.1,"m\u002Fs",[542],"v = √(9.8 × 75).","v = √(9.8 × 75) ≈ **27.1 m\u002Fs**.",[545,546],"formulas","square roots",{"id":548,"section":27,"level":132,"prompt":549,"check":550,"hints":553,"solution":555,"skills":556},"tides.q033","At Chandipur, the sea retreats about 5 km at low tide for a total fall in water level of about 3 m. What is the slope of the beach, expressed as '1 metre down for every ___ metres out'?",{"kind":90,"answer":551,"tolerance":552},1667,20,[554],"Convert 5 km to metres, then divide by 3 m.","5 km = 5000 m. 5000 ÷ 3 ≈ **1 in 1667** — a tiny slope, which is why a small fall in water level uncovers so much beach.",[449,557],"unit conversion",{"id":559,"section":27,"level":132,"prompt":560,"check":561,"hints":565,"solution":567,"skills":568},"tides.q034","A basin resonates strongly when its length is close to a quarter of the tidal wavelength, L = v × T ÷ 4. Using v ≈ 27.1 m\u002Fs (Bay of Fundy) and the M2 period T = 12 h 25 min in seconds (≈44,712 s), find the resonant length in km.",{"kind":90,"answer":562,"tolerance":563,"unit":564},303,15,"km",[566],"L = 27.1 × 44,712 ÷ 4, then convert metres to km.","L = 27.1 × 44,712 ÷ 4 ≈ 303359 m ≈ **303 km**, close to the real Gulf of Maine–Bay of Fundy system's size.",[545,351],{"id":570,"section":27,"level":147,"prompt":571,"check":572,"hints":576,"solution":578,"skills":579},"tides.q035","Two gulfs are both closed at one end and open at the other. Gulf X is 45 m deep on average; Gulf Y is only 8 m deep. Both are 220 km long. Using v = √(g × depth) and the resonant condition L ≈ v × T ÷ 4 (T = M2 period), explain which gulf is closer to resonance and why.",{"kind":135,"accept":573},[574,575],"gulf x is closer because its greater depth gives a faster wave speed and a resonant length nearer 220 km, gulf y's resonant length is much shorter","the deeper gulf x is closer to resonance since its wave speed is higher giving a resonant length near 220 km",[577],"Work out v for each depth, then the resonant length for each, and compare both with 220 km.","Gulf X: v = √(9.8×45) ≈ 21.0 m\u002Fs, giving a resonant length ≈ 21.0×44,712÷4 ≈ 235 km — close to 220 km. Gulf Y: v = √(9.8×8) ≈ 8.9 m\u002Fs, giving a resonant length ≈ 8.9×44,712÷4 ≈ 99 km — far short of 220 km. **Gulf X is much closer to resonance** and should show a noticeably bigger tidal range.",[144,351],{"id":581,"section":31,"level":45,"prompt":582,"check":583,"hints":594,"solution":596,"skills":597},"tides.q036","A coast with just one high tide and one low tide a day has a…",{"kind":48,"options":584,"correct":593},[585,587,589,591],{"id":51,"label":586},"Semidiurnal tide",{"id":54,"label":588},"Diurnal tide",{"id":57,"label":590},"Mixed tide",{"id":60,"label":592},"Spring tide",[54],[595],"'Di' means two; this is the opposite pattern.","A **diurnal** tide: just one high and one low water each lunar day, as on the Gulf of Mexico.",[31],{"id":599,"section":31,"level":45,"prompt":600,"check":601,"hints":612,"solution":614,"skills":615},"tides.q037","What does a tide table show?",{"kind":48,"options":602,"correct":611},[603,605,607,609],{"id":51,"label":604},"Only the weather forecast",{"id":54,"label":606},"The predicted times and heights of high and low water for a port",{"id":57,"label":608},"The temperature of the sea",{"id":60,"label":610},"Only tsunami warnings",[54],[613],"It is used by fishers and sailors every day.","A **tide table** lists the predicted times and heights of high and low water at a particular port, for every day of the year.",[616],"tide-tables",{"id":618,"section":31,"level":102,"prompt":619,"check":620,"hints":628,"solution":630,"skills":631},"tides.q038","Two high tides a day, but one clearly bigger than the other, is called a…",{"kind":48,"options":621,"correct":627},[622,623,624,625],{"id":51,"label":588},{"id":54,"label":586},{"id":57,"label":590},{"id":60,"label":626},"Neap tide",[57],[629],"Two of each, but unequal.","A **mixed tide**: two highs and two lows, like a semidiurnal tide, but noticeably unequal — as at San Francisco Bay.",[31],{"id":633,"section":31,"level":102,"prompt":634,"check":635,"hints":637,"solution":639,"skills":640},"tides.q039","Low water is 1.0 m and high water is 7.0 m. Using the rule of twelfths, how high is the water two hours after low water?",{"kind":90,"answer":636,"tolerance":92,"unit":93},3,[638],"Range = 6.0 m. Two hours brings 1 + 2 = 3 twelfths of the range.","Range = 7.0 − 1.0 = 6.0 m. After two hours, 3 twelfths have risen: 6.0 × 3 ÷ 12 = 1.5 m. Water level = 1.0 + 1.5 = **3.0 m**.",[641,351],"rule of twelfths",{"id":643,"section":31,"level":102,"prompt":644,"check":645,"hints":656,"solution":658,"skills":659},"tides.q040","Why can a tide table be printed years in advance and still be accurate?",{"kind":48,"options":646,"correct":655},[647,649,651,653],{"id":51,"label":648},"Tides never change",{"id":54,"label":650},"Harmonic analysis breaks a measured tide into steady waves tied to the predictable Moon and Sun, which can be projected forward",{"id":57,"label":652},"It is re-measured every single day",{"id":60,"label":654},"Computers guess from recent weather",[54],[657],"Think about what harmonic analysis actually does with old tide-gauge data.","**Harmonic analysis** finds the steady, repeating waves (constituents) hidden in a port's past tides, tied to the predictable motion of the Moon and Sun — and those can be run forward to any future date.",[660],"harmonic analysis",{"id":662,"section":31,"level":132,"prompt":663,"check":664,"hints":675,"solution":677,"skills":678},"tides.q041","A boat needs 3.0 m of water to float. Low water is 0.6 m at 08:00 and high water is 5.0 m at 14:15. Using the rule of twelfths, has the boat enough water at 11:00 (three hours after low water)?",{"kind":48,"options":665,"correct":674},[666,668,670,672],{"id":51,"label":667},"Yes, there is exactly enough",{"id":54,"label":669},"No, there is about 0.2 m short of enough",{"id":57,"label":671},"Yes, there is 1 m more than enough",{"id":60,"label":673},"No, the water is still falling at 11:00",[54],[676],"After three hours, 1+2+3=6 twelfths (half the range) have risen.","Range = 5.0 − 0.6 = 4.4 m. After 3 hours, half the range (2.2 m) has risen: 0.6 + 2.2 = 2.8 m — **about 0.2 m short** of the 3.0 m the boat needs.",[641,351],{"id":680,"section":31,"level":147,"prompt":681,"check":682,"hints":686,"solution":688,"skills":689},"tides.q042","Explain the standard tidal form factor F = (K1 + O1) ÷ (M2 + S2), and classify a port with M2=60, S2=20, K1=20, O1=20.",{"kind":135,"accept":683},[684,685],"F is diurnal over semidiurnal amplitude ratio, here F=40\u002F80=0.5, mixed mainly semidiurnal","0.5, mixed mainly semidiurnal",[687],"Add the two diurnal constituents, add the two semidiurnal ones, and divide.","F compares the size of the two daily waves (K1, O1) with the two twice-daily waves (M2, S2). Here F = (20+20) ÷ (60+20) = 40 ÷ 80 = **0.50**, which is **mixed, mainly semidiurnal** (0.25 ≤ F \u003C 1.5).",[660,449],{"id":691,"section":35,"level":45,"prompt":692,"check":693,"hints":704,"solution":706,"skills":707},"tides.q043","Why do many small fishing boats need to leave and return near high tide?",{"kind":48,"options":694,"correct":703},[695,697,699,701],{"id":51,"label":696},"Fish only bite at high tide",{"id":54,"label":698},"There is enough water under the boat to float at high tide",{"id":57,"label":700},"It is a tradition with no practical reason",{"id":60,"label":702},"High tide is always calmer",[54],[705],"Think about what happens to a boat at low tide in a shallow creek.","At low tide a shallow creek can leave a boat sitting on the mud; only near **high tide** is there enough water under the hull to float.",[35],{"id":709,"section":35,"level":45,"prompt":710,"check":711,"hints":722,"solution":724,"skills":725},"tides.q044","What does a tidal barrage do?",{"kind":48,"options":712,"correct":721},[713,715,717,719],{"id":51,"label":714},"Stops the tide completely",{"id":54,"label":716},"Uses the rise and fall of the tide to generate electricity",{"id":57,"label":718},"Only protects against storm surges",{"id":60,"label":720},"Predicts tsunamis",[54],[723],"Think of a dam across a bay with turbines.","A **tidal barrage** is a dam with turbines across a bay or estuary, generating electricity as water flows in and out with the tide.",[726],"tidal power",{"id":728,"section":35,"level":102,"prompt":729,"check":730,"hints":741,"solution":743,"skills":744},"tides.q045","Why is a storm surge especially dangerous if it arrives at high tide?",{"kind":48,"options":731,"correct":740},[732,734,736,738],{"id":51,"label":733},"High tide causes storms",{"id":54,"label":735},"The surge adds on top of the already-high sea level",{"id":57,"label":737},"Storms only happen at high tide",{"id":60,"label":739},"It makes no real difference",[54],[742],"A storm surge rides on top of whatever the tide is already doing.","The surge adds **on top of** the ordinary tide, so the same surge reaches a far higher total level at high tide than at low tide — as the 1999 Odisha Super Cyclone showed.",[745],"storm surge",{"id":747,"section":35,"level":102,"prompt":748,"check":749,"hints":760,"solution":762,"skills":763},"tides.q046","What two conditions does a tidal bore need to form?",{"kind":48,"options":750,"correct":759},[751,753,755,757],{"id":51,"label":752},"Cold water and a full moon",{"id":54,"label":754},"A large tidal range and a shallowing, funnel-shaped river mouth",{"id":57,"label":756},"An earthquake under the river",{"id":60,"label":758},"A very deep, wide river with no narrowing",[54],[761],"Think about the Hooghly and the Qiantang.","A **large tidal range** forced into a **shallowing, narrowing river mouth**, so the deeper water behind the tide's front catches up with the shallower water ahead of it.",[764],"tidal bore",{"id":766,"section":35,"level":132,"prompt":767,"check":768,"hints":771,"solution":773,"skills":774},"tides.q047","The Qiantang River's tidal bore reaches about 9 m and travels at up to about 40 km\u002Fh. The Hooghly's typical bore is often over 2.1 m. Roughly how many times taller is the Qiantang's typical bore than the Hooghly's typical figure?",{"kind":90,"answer":769,"tolerance":455,"unit":770},4.3,"times",[772],"Divide the Qiantang figure by the Hooghly figure.","9 ÷ 2.1 ≈ **4.3 times** taller — the Qiantang is the world's largest tidal bore, the Hooghly's is real but modest by comparison.",[449],{"id":776,"section":35,"level":132,"prompt":777,"check":778,"hints":782,"solution":784,"skills":785},"tides.q048","A tide front in water 4 m deep travels at about 6.27 m\u002Fs, and a front stuck in water 1 m deep travels at about 3.13 m\u002Fs. If the deeper front starts 20 km behind the shallower one, roughly how many hours before it catches up?",{"kind":90,"answer":779,"tolerance":780,"unit":781},6.4,0.6,"h",[783],"Time = distance ÷ (difference in speed). Convert the speed difference to km\u002Fh first.","Speed difference ≈ 11.3 km\u002Fh. Time = 20 km ÷ 11.3 km\u002Fh ≈ **6.4 hours**.",[351,786],"relative speed",{"id":788,"section":35,"level":147,"prompt":789,"check":790,"hints":794,"solution":796,"skills":797},"tides.q049","Explain why a tidal barrage is only worth building at a few coastlines in the world, using the Rance (France, 240 MW, 1966) and Sihwa Lake (South Korea, 254 MW, 2011) as your evidence, and connect this to why the Gulf of Khambhat and Gulf of Kutch are the sites most studied in India.",{"kind":135,"accept":791},[792,793],"both real barrages sit where the tidal range is unusually large, and khambhat and kutch have india's largest ranges from funnelling and resonance","they need an unusually large tidal range like khambhat and kutch have",[795],"Think about what both real stations have in common about their location.","Both real barrages were built where the tidal range is **unusually large** (funnelling and\u002For resonance), because a barrage only generates significant power when a large volume of water moves a large height difference. The Gulf of Khambhat and Gulf of Kutch have India's largest tidal ranges for exactly the same reasons (funnel shape, shallowing), which is why they are the sites most studied for Indian tidal power.",[144,726],{"id":799,"section":39,"level":45,"prompt":800,"check":801,"hints":812,"solution":814,"skills":815},"tides.q050","Tidal friction is slowly doing what to Earth's day?",{"kind":48,"options":802,"correct":811},[803,805,807,809],{"id":51,"label":804},"Shortening it",{"id":54,"label":806},"Lengthening it",{"id":57,"label":808},"Nothing; it stays exactly the same",{"id":60,"label":810},"Making it shorter, then longer, at random",[54],[813],"Think about what a brake does to a spinning object.","Tidal friction is very slowly **lengthening** Earth's day, as the Moon's tidal bulge acts as a brake on Earth's spin.",[816],"tidal friction",{"id":818,"section":39,"level":45,"prompt":819,"check":820,"hints":823,"solution":825,"skills":826},"tides.q051","About how fast is the Moon receding from Earth?",{"kind":90,"answer":821,"tolerance":455,"unit":822},3.83,"cm\u002Fyear",[824],"It is measured by laser ranging.","About **3.8 cm per year**, measured by timing laser pulses bounced off mirrors left on the Moon by Apollo astronauts.",[827],"moon recession",{"id":829,"section":39,"level":102,"prompt":830,"check":831,"hints":834,"solution":836,"skills":837},"tides.q052","Fossil evidence from 70-million-year-old rudist bivalves suggests a Cretaceous year held about 372 days. Using a year of 8765.8 hours, how long was a Cretaceous day, in hours (one decimal place)?",{"kind":90,"answer":832,"tolerance":833,"unit":781},23.6,0.1,[835],"Hours per day = hours per year ÷ days per year.","8765.8 ÷ 372 ≈ **23.6 hours** — shorter than today's 24-hour day, as expected.",[351],{"id":839,"section":39,"level":102,"prompt":840,"check":841,"hints":852,"solution":854,"skills":855},"tides.q053","What is 'angular momentum', and why does tidal friction not destroy it?",{"kind":48,"options":842,"correct":851},[843,845,847,849],{"id":51,"label":844},"A type of energy that disappears over time",{"id":54,"label":846},"A measure of spinning\u002Forbiting motion that can only be transferred, not destroyed, within a system",{"id":57,"label":848},"The speed of a tsunami",{"id":60,"label":850},"A unit of tidal range",[54],[853],"Where does Earth's 'lost' spin actually go?","**Angular momentum** measures spinning or orbiting motion, and in an isolated system it can only move from one part to another. Earth's spin slows while the Moon's orbit widens — the momentum is transferred, not destroyed.",[856],"angular momentum",{"id":858,"section":39,"level":132,"prompt":859,"check":860,"hints":864,"solution":866,"skills":867},"tides.q054","Using the recession rate of 3.8 cm\u002Fyear, roughly how many kilometres has the Moon receded over the last 100 million years, and why is this figure only a rough estimate rather than a fact?",{"kind":135,"accept":861},[862,863],"about 3800 km, rough because tidal friction depends on ocean shapes which have changed hugely over that time so the rate was not constant","roughly 3800 km, but it is a rough extrapolation since continents and ocean basins have changed",[865],"Multiply the rate by 100 million years and convert to km; then think about what might have changed over that time.","3.8 cm × 100,000,000 ≈ 383000000 cm ≈ **3830 km**. It is only a rough estimate because tidal friction depends on ocean basin shapes and depths, which have changed enormously over 100 million years of continental drift — today's exact rate has not held constant that whole time.",[351,144],{"id":869,"section":39,"level":132,"prompt":870,"check":871,"hints":882,"solution":884,"skills":885},"tides.q055","Why does Io, a moon of Jupiter, have constant volcanic eruptions?",{"kind":48,"options":872,"correct":881},[873,875,877,879],{"id":51,"label":874},"It is closest to the Sun",{"id":54,"label":876},"An orbital resonance with Europa and Ganymede keeps stretching its orbit, so Jupiter's gravity keeps flexing and heating it",{"id":57,"label":878},"It has a very thick atmosphere",{"id":60,"label":880},"It spins extremely fast on its axis",[54],[883],"Io, Europa and Ganymede orbit in a 1:2:4 ratio.","The **Laplace resonance** (orbital periods close to 1:2:4 with Europa and Ganymede) keeps Io's orbit from settling into a neat circle, so Jupiter's gravity never stops flexing and heating it.",[886],"tidal heating",{"id":888,"section":39,"level":147,"prompt":889,"check":890,"hints":901,"solution":903,"skills":904},"tides.q056","Devonian corals (about 380 million years ago) and Ediacaran tidal rhythmites (about 620 million years ago) both suggest a day of close to 21.9 hours, despite 240 million years between them. What does recent research suggest explains this near-match, given that steady lengthening should have made the Devonian day noticeably longer?",{"kind":48,"options":891,"correct":900},[892,894,896,898],{"id":51,"label":893},"Both measurements are simply wrong",{"id":54,"label":895},"Day-length increase 'stalled' for about a billion years: an atmospheric thermal tide cancelled lunar tidal friction",{"id":57,"label":897},"The Moon stopped moving during that entire interval",{"id":60,"label":899},"Earth's orbit around the Sun changed shape instead",[54],[902],"If lengthening were steady, would 240 million more years of it produce a longer or shorter day than the older figure?","**No** — steady lengthening over 240 more million years should make the Devonian day noticeably longer than the Ediacaran one, not nearly identical. Recent research proposes that day-length increase **stalled** for roughly a billion years in the middle of Earth's history, because a Sun-driven **atmospheric thermal tide** pushed back against lunar tidal friction almost exactly hard enough to cancel it.",[144,905],"history of science",{"id":907,"section":11,"level":102,"prompt":908,"check":909,"hints":920,"solution":922,"skills":923},"tides.q057","A rock pool is full of small creatures when you visit at 3pm and completely dry, open to the air, when you return at 9pm. Which state of the tide was it at 3pm?",{"kind":48,"options":910,"correct":919},[911,913,915,917],{"id":51,"label":912},"High tide",{"id":54,"label":914},"Low tide",{"id":57,"label":916},"Halfway through the flood",{"id":60,"label":918},"Impossible to tell",[51],[921],"Rock pools are only 'full and part of the sea' at one state of the tide.","**High tide** — rock pools are simply part of the sea at high water, and are left behind, open to the air, once the tide falls.",[144],{"id":925,"section":15,"level":102,"prompt":926,"check":927,"hints":932,"solution":934,"skills":935},"tides.q058","Complete the sentence: the tidal bulges stay roughly lined up with the Moon while Earth ___.",{"kind":135,"accept":928},[929,930,931],"turns underneath them","spins through them","rotates underneath the bulges",[933],"Which one is doing the moving: the bulges, or the planet?","Earth **turns underneath them** — the bulges stay pointing towards (and away from) the Moon, while the spinning planet carries each coast through them in turn.",[176],{"id":937,"section":19,"level":102,"prompt":938,"check":939,"hints":941,"solution":943,"skills":944},"tides.q059","How many high tides does most of India's coast experience in one lunar day?",{"kind":90,"answer":415,"tolerance":940},0,[942],"Earth turns through both bulges once per lunar day.","**Two** — the two-bulge picture gives two highs and two lows in each 24 h 50 min lunar day.",[290],{"id":946,"section":23,"level":102,"prompt":947,"check":948,"hints":959,"solution":961,"skills":962},"tides.q060","Match: which Moon phase pairs with a NEAP tide?",{"kind":48,"options":949,"correct":958},[950,952,954,956],{"id":51,"label":951},"New moon",{"id":54,"label":953},"First quarter",{"id":57,"label":955},"Full moon",{"id":60,"label":957},"None of these",[54],[960],"Neap tides are at the quarters, not new\u002Ffull moon.","**First quarter** (and also last quarter) — the Sun sits at right angles to the Moon, partly cancelling its tide.",[23],{"id":964,"section":27,"level":102,"prompt":965,"check":966,"hints":970,"solution":972,"skills":973},"tides.q061","List the typical tidal ranges in increasing order: Kochi, Mumbai, Bhavnagar.",{"kind":135,"accept":967},[968,969],"kochi, mumbai, bhavnagar","kochi then mumbai then bhavnagar",[971],"About 1 m, 4.4 m and 10 m respectively.","**Kochi (≈1 m), then Mumbai (≈4.4 m), then Bhavnagar (≈10 m)** — an open coast, then a broad shelf, then a narrowing gulf.",[497],{"id":975,"section":31,"level":102,"prompt":976,"check":977,"hints":988,"solution":990,"skills":991},"tides.q062","A port's tide table shows only one high water and one low water most days. What tide type is this?",{"kind":48,"options":978,"correct":987},[979,981,983,985],{"id":51,"label":980},"Semidiurnal",{"id":54,"label":982},"Diurnal",{"id":57,"label":984},"Mixed",{"id":60,"label":986},"Neap",[54],[989],"Count the highs and lows per day.","**Diurnal** — one high, one low, each lunar day.",[31],{"id":993,"section":35,"level":102,"prompt":994,"check":995,"hints":1006,"solution":1008,"skills":1009},"tides.q063","Why do salt makers in Gujarat time the flooding of their pans to a spring tide rather than any ordinary high tide?",{"kind":48,"options":996,"correct":1005},[997,999,1001,1003],{"id":51,"label":998},"Spring tides taste saltier",{"id":54,"label":1000},"A spring tide floods the pan higher, giving more water to evaporate before the next good chance a fortnight later",{"id":57,"label":1002},"It has no real reason, it is just tradition",{"id":60,"label":1004},"Spring tides only happen once a year",[54],[1007],"Think about how much water a bigger tide can deliver, and how long until the next spring tide.","A spring tide floods the pan **higher**, delivering more water in one filling; missing it means waiting about a fortnight for the next spring tide.",[35],{"id":1011,"section":39,"level":102,"prompt":1012,"check":1013,"hints":1024,"solution":1026,"skills":1027},"tides.q064","What evidence do scientists use to estimate day length hundreds of millions of years ago?",{"kind":48,"options":1014,"correct":1023},[1015,1017,1019,1021],{"id":51,"label":1016},"Old written records",{"id":54,"label":1018},"Fine daily growth ridges counted between yearly bands in fossils and sediment",{"id":57,"label":1020},"Satellite photographs",{"id":60,"label":1022},"It cannot be estimated at all",[54],[1025],"Think of fossil shells and tidal rhythmites.","**Fine daily growth ridges**, counted between clear yearly (or monthly) markers in fossils or sediment layers, directly count how many days made up a year at that time.",[1028],"fossil evidence",{"id":1030,"section":11,"level":132,"prompt":1031,"check":1032,"hints":1036,"solution":1038,"skills":1039},"tides.q065","A fisher says: 'the tide is calm today, so there won't be any waves.' Explain what is wrong with this reasoning.",{"kind":135,"accept":1033},[1034,1035],"tide and waves are independent, a calm tidal day can still have wind waves and vice versa","they are unrelated, waves come from wind not the tide's size",[1037],"Are tides and waves caused by the same thing?","Tides and waves are **independent**: a tide's size depends on the Moon and Sun, while waves depend on the wind. A calm (small) tide can still have big wind-driven waves, and vice versa.",[129,144],{"id":1041,"section":15,"level":132,"prompt":1042,"check":1043,"hints":1047,"solution":1049,"skills":1050},"tides.q066","A student says 'the Moon lifts the ocean straight up, like a hand pulling up a tablecloth.' Using the actual size of the effect, explain why this picture is wrong.",{"kind":135,"accept":1044},[1045,1046],"the straight up pull is only about a ten millionth of gravity, far too weak to lift water; it mostly pushes water sideways which piles up over hours","the vertical pull is tiny (about 1e-7 of g), the real motion is mostly sideways flow that piles up at the coasts",[1048],"The straight-up part of the Moon's pull is only about a ten-millionth of Earth's own gravity.","The straight-up part of the Moon's pull is only about a **ten-millionth** of Earth's gravity — far too weak to lift anything. The real mechanism is water sliding **sideways** along the surface for hours, piling up at the coasts, which is a much easier thing for a weak force to do.",[144,129],{"id":1052,"section":23,"level":132,"prompt":1053,"check":1054,"hints":1056,"solution":1058,"skills":1059},"tides.q067","The Moon's tide-raising effect is about 2.18 times the Sun's. Show that the Sun's share of a spring tide (Moon + Sun combined) is about 31%.",{"kind":90,"answer":1055,"tolerance":415,"unit":416},31,[1057],"Sun's share = 1 ÷ (Moon units + Sun's 1 unit) × 100.","Sun's share = 1 ÷ (2.18 + 1) × 100 ≈ **31%**.",[260,449],{"id":1061,"section":31,"level":132,"prompt":1062,"check":1063,"hints":1066,"solution":1068,"skills":1069},"tides.q068","A port has constituents M2=100, S2=30, K1=10, O1=8. Calculate its form factor F and classify its tide type.",{"kind":90,"answer":1064,"tolerance":1065},0.14,0.02,[1067],"F = (K1+O1) ÷ (M2+S2).","F = (10+8) ÷ (100+30) = 18 ÷ 130 ≈ **0.14** — **semidiurnal**, since F \u003C 0.25.",[660,449],{"id":1071,"section":27,"level":132,"prompt":1072,"check":1073,"hints":1084,"solution":1086,"skills":1087},"tides.q069","The open ocean averages about 3700 m deep. Using v = √(g × depth), a tide travels at roughly 686 km\u002Fh there. Why does this still count as a 'shallow-water' wave?",{"kind":48,"options":1074,"correct":1083},[1075,1077,1079,1081],{"id":51,"label":1076},"Because the open ocean is not actually very deep",{"id":54,"label":1078},"Because 'shallow' compares depth with wavelength, and a tidal wave's length (thousands of km) vastly exceeds any ocean depth",{"id":57,"label":1080},"Because shallow-water waves are always fast",{"id":60,"label":1082},"It does not; the formula is only an approximation",[54],[1085],"'Shallow' compares depth with wavelength, not with an absolute number.","v = √(9.8 × 3700) ≈ 191 m\u002Fs ≈ **686 km\u002Fh**. It still counts as 'shallow' because a tidal wave's length is thousands of kilometres, utterly dwarfing even a deep ocean's depth.",[545,144],{"id":1089,"section":35,"level":147,"prompt":1090,"check":1091,"hints":1102,"solution":1104,"skills":1105},"tides.q070","A coastal planner must choose between a tidal barrage in a 9 m-range gulf or a 2 m-range gulf, all else being similar. Which statement best captures the full picture from this topic?",{"kind":48,"options":1092,"correct":1101},[1093,1095,1097,1099],{"id":51,"label":1094},"The 2 m gulf is better, because a smaller range means calmer, easier construction",{"id":54,"label":1096},"The 9 m gulf is the far better energy site, but non-energy factors (mudflats, fisheries, shipping) could still rule it out",{"id":57,"label":1098},"Range makes no difference to a tidal barrage's output",{"id":60,"label":1100},"Either site would generate exactly the same amount of electricity",[54],[1103],"Real barrages (Rance, Sihwa) only get built where the range is unusually large — but energy is not the only factor.","The **9 m gulf** is the far better energy candidate, since both real barrages (Rance, Sihwa) were only worth building where the range was unusually large. But a non-energy factor — harm to mudflats, fisheries, or shipping routes — could still make it the wrong choice overall.",[144,726],{"id":1107,"section":39,"level":147,"prompt":1108,"check":1109,"hints":1120,"solution":1122,"skills":1123},"tides.q071","The Moon's rotation period (27.32 days) already exactly equals its orbital period (synchronous rotation), reached through tidal braking by Earth. What would eventually happen to Earth's own spin given enough time, and why is this unlikely to actually be reached?",{"kind":48,"options":1110,"correct":1119},[1111,1113,1115,1117],{"id":51,"label":1112},"Earth's spin would speed up until it matched the Moon's orbit; but the Moon will run out of angular momentum first",{"id":54,"label":1114},"Earth's day would lengthen to match the month (mutual tidal locking); but the Sun will become a red giant long before that finishes",{"id":57,"label":1116},"Nothing would ever change; Earth's spin is already stable",{"id":60,"label":1118},"Earth's spin would stop completely within a few million years",[54],[1121],"Pluto and Charon show what the 'finished' version of this process looks like.","Given enough time, Earth's day would lengthen until it matched the month, reaching **mutual tidal locking** (as Pluto and Charon already show). This is very unlikely to actually happen, because the Sun is expected to swell into a red giant in a few billion years, transforming the Earth–Moon system long before the tidal-locking process could finish.",[144,856],[1125,1126,1127,1128,1129,1130,1131],"tides-noaa-tides-tutorial","tides-nasa-moon-tides","tides-wikipedia-tide","tides-wikipedia-bay-of-fundy","tides-incois-tide-forecasting","tides-survey-of-india-tidal","tides-wikipedia-tidal-acceleration","needs_review",{"generatedBy":1134,"notes":1135},"claude-code","Draft generated locally; pending owner review.","c0df014a5d398f467807c76e6dfd2e3a278da63b2a23a3aa36dd7e43aa4251c0",{"logic:questions":1138,"source:tides-incois-tide-forecasting":1139,"source:tides-nasa-moon-tides":1140,"source:tides-noaa-tides-tutorial":1141,"source:tides-survey-of-india-tidal":1142,"source:tides-wikipedia-bay-of-fundy":1143,"source:tides-wikipedia-tidal-acceleration":1144,"source:tides-wikipedia-tide":1145},"e7fd7c240a65bea1bff6277f7af7cca65e2c7fcb13c6d756cb943d53f3cbc948","c8bcfd0dc5b9671b4b89f5c1abc1f9dbf9e3f3caf1325102661e5d41a455803f","d71a4932c5e1df6475bdbf45fedc9a0a348ce79ce9d887d3112d5b033fe39998","82683db912324c1f40c9e2f4d4cc0312c637b4af2490f13ca404549ec5e414a0","2e99068442e3818ea49964d49796e478ce8718d4e7d9cb9138f2efc27ba39702","a11570fb567503cb357092d9ba5ad5cc3f29fa50d9094d46c5a4b2435e012a92","892b3b28f4a10515451eae991092c14ee1276c8074bf745241e15c5e6c826b8a","0c80de822f7941587e525d4a3ae84aabe44b7c2a5d1851e5885d69db0da1e874","preview-7e1cbbcc4f",1789899599995]