Waves and tides

Correct answer: B. Crest

The governing concept is the basic anatomy of a wave. A crest is the highest point of a wave measured relative to the mean or undisturbed water level, so option B is correct. The trough is the lowest point below that mean level. Wavelength is a horizontal distance between corresponding points on successive waves, while amplitude measures the maximum displacement from the mean level; neither is listed as the correct high point here. Tide is a periodic rise and fall of sea level, and current is a general flow of water, so both describe broader water movements rather than a part of one wave. Remembering crest as the raised part and trough as the depressed part prevents the common reversal.

Correct answer: C. Trough

The governing concept is wave structure measured around a mean water level. The trough is the lowest point of a wave below the average or undisturbed level, so option C is correct. The crest is the corresponding highest point. Wavelength is not a vertical part at all; it is the horizontal distance between two successive crests or two successive troughs. Amplitude is the maximum vertical displacement from the mean level, not the name of the lowest point itself. Thus the definitions distinguish all four terms: crest means high point, trough means low point, wavelength means horizontal spacing, and amplitude means displacement. This terminology is useful for describing wave height, which is measured vertically from crest to trough.

Correct answer: D. Between crest and trough

The governing concept is the vertical measurement of wave height. Wave height is the vertical distance from the crest, the highest point of a wave, down to the adjacent trough, the lowest point. Therefore option D is correct. The distance between two crests is wavelength, which is a horizontal measure of one complete wave cycle, not wave height. Distances between coasts or rivers describe geographical separation and have no standard role in defining the height of a wave. Amplitude is related but is measured from the mean water level to a crest or trough; for a symmetrical wave, wave height is approximately twice the amplitude. The key distinction is crest-to-trough vertical distance versus crest-to-crest horizontal distance.

Correct answer: A. Wavelength

The governing concept is wavelength, a basic measurement of periodic waves. Wavelength is the horizontal distance between two successive points in the same phase, such as two consecutive crests or two consecutive troughs. Therefore option A is correct. A tide is a periodic rise and fall of sea level, not a distance. A trough is the low point of an individual wave, while a coast is the boundary between land and sea. The question specifies two successive crests, so it refers to the repeating horizontal interval of the wave pattern. This differs from wave height, which is the vertical distance from crest to trough, and from amplitude, which is the displacement from the mean water level to a crest or trough. These distinctions help avoid confusing horizontal and vertical wave measurements.

Correct answer: B. Gravitational force

The main cause of tides is gravitational force, especially the gravitational attraction of the Moon. The Sun also contributes because it is very massive, although its much greater distance makes its tidal influence weaker than the Moon’s. Gravity produces two broad tidal bulges: one on the side of Earth facing the Moon and another on the opposite side, related to the Earth–Moon system’s motion. As Earth rotates through these bulges, coastal water levels rise and fall, producing high and low tides. Friction, sound and electric force do not create the regular ocean-wide tidal pattern. Therefore, option B is correct.

Correct answer: C. Moon and Sun

Tides are mainly influenced by the Moon and the Sun. The Moon is the stronger influence because it is much closer to Earth, so its gravitational attraction produces the dominant tidal effect. The Sun also exerts a significant tidal force because of its enormous mass. When the Sun, Earth and Moon are nearly aligned, their effects reinforce one another and produce spring tides with a larger tidal range. When the Sun and Moon are approximately at right angles, their effects partly counteract one another and neap tides occur. The other listed planets are too distant and have no comparable influence on Earth’s tides. Thus, option C is correct.

Correct answer: A. Raised above the normal level

High tide is the stage of the tidal cycle when sea level at a particular coast rises above its usual or average level. This rise results mainly from the gravitational effects of the Moon and, to a lesser extent, the Sun, together with the motion of the Earth–Moon system. The exact height of high tide varies with coastline shape, ocean-floor relief and the position of the celestial bodies. Option B describes low tide, when the water level is relatively lower and more of the shore may be exposed. Options C and D do not describe normal tidal conditions. Therefore, option A is correct.

Correct answer: A. Lower than its usual level

Low tide is the part of the tidal cycle when the sea level at a coastal location falls below its usual or average level. The apparent recession of water can expose rocks, sand or a wider strip of shore. This change is produced by the regular redistribution of ocean water under the gravitational influence of the Moon and Sun, not simply by ordinary wind-generated waves. Option B describes high tide, when the water level is elevated. Option C is incorrect because tides involve a measurable rise and fall, while option D confuses tides with unusually large waves or storm conditions. Therefore, option A is correct.

Correct answer: B. Difference between high- and low-tide levels

Correct answer: B. Tidal range is the vertical difference in water level between high tide and low tide at a particular place. To calculate it, subtract the low-tide level from the high-tide level. For example, if high tide reaches 5 metres above a reference level and low tide reaches 1 metre above that same reference level, the tidal range is 5 − 1 = 4 metres. It is not the total depth of the sea, because depth measures how far the seabed lies below the water surface. It is not the sound of waves or a colour difference. Tidal range varies from place to place because coastline shape, basin form, seabed relief, and the positions of the Moon and Sun affect tides. Memory cue: tidal range means high-water mark minus low-water mark.

Correct answer: C. When the Sun, Earth and Moon are nearly aligned

A spring tide occurs when the Sun, Earth and Moon are nearly aligned, which happens around the new-moon and full-moon phases. In this arrangement, the gravitational effects of the Moon and Sun act along nearly the same line and reinforce one another. The result is a greater difference between high tide and low tide: high tides are relatively higher and low tides relatively lower. The word “spring” refers to the strong springing or rising of the tidal range, not to the spring season. Wind, river floods and a drying sea are not the astronomical conditions that define spring tide. Therefore, option C is correct.

Correct answer: D. New moon and full moon

Spring tides are associated with the new moon and full moon. During both phases, the Sun, Earth and Moon are approximately aligned, although their order differs: at new moon the Moon is between the Sun and Earth, while at full moon the Earth lies between the Sun and Moon. The lunar and solar tidal forces then reinforce one another, producing a larger tidal range than usual. First- and third-quarter phases place the Sun and Moon roughly at right angles and are linked with weaker neap tides. A crescent or daytime alone does not determine the spring-tide condition. Therefore, option D is correct.

Correct answer: A. When the Sun and Moon act at right angles

A neap tide occurs when the gravitational influences of the Sun and Moon act approximately at right angles to one another, as happens around the first-quarter and third-quarter Moon. Their tidal effects then partly counteract rather than reinforce each other. Consequently, the difference between high tide and low tide becomes smaller than the average, producing a relatively low tidal range. Option C is not correct because a storm may raise water temporarily but is not the astronomical cause of a neap tide. Options B and D describe impossible or unrelated conditions; gravity remains essential. Therefore, option A is correct.

Correct answer: B. First and third quarter

Neap tides are generally associated with the first-quarter and third-quarter Moon phases. At these phases, the Sun and Moon appear approximately at right angles as viewed from Earth. Their gravitational tidal effects therefore act in different directions and partly offset one another. The result is a smaller difference between high tide and low tide, known as a lower tidal range. In contrast, new moon and full moon place the Sun, Earth and Moon nearly in line and produce spring tides with a greater range. The terms “first” and “third” quarter refer to lunar phases, not quarters of the year. Hence, option B is correct.

Correct answer: C. High

The tidal range during a spring tide is relatively high. Tidal range is calculated as the difference between the water level at high tide and the water level at low tide. Around the new moon and full moon, the Sun, Earth and Moon are nearly aligned, so the lunar and solar tidal forces reinforce one another. This produces higher-than-usual high tides and lower-than-usual low tides, making the difference between them large. For example, if high tide is 6 metres and low tide is 1 metre relative to the same datum, the range is 6 − 1 = 5 metres. Therefore, option C is correct.

Correct answer: D. Low

Tidal range means the vertical difference between the water level at high tide and the water level at the following low tide. During a neap tide, the Sun, Earth, and Moon are approximately at right angles, so the gravitational effects of the Sun and Moon partly oppose each other. Their combined tidal force is therefore weaker than during a spring tide. As a result, high tides are relatively lower and low tides are relatively higher, producing a small or low tidal range. Option D is correct. Option A describes spring-tide conditions, while B is scientifically meaningless and C does not identify the characteristic feature of neap tide.

Correct answer: A. Because the Moon is close to Earth

The Moon produces a noticeable tidal effect mainly because it is much closer to Earth than the Sun. Tides result from differences in gravitational pull across Earth, not simply from the total strength of gravity. Although the Sun is vastly larger and also raises tides, its great distance makes its differential tidal effect on Earth weaker than the Moon's. The Moon's attraction creates a water bulge on the side facing it, while another bulge develops on the opposite side because of the Earth-Moon system's motion. Therefore option A is correct. Option B is false because the Moon is much smaller than the Sun; C is false because the Sun has gravity; D is physically impossible.

Correct answer: B. Energy

A sea wave is primarily a moving form of energy travelling through the water. Wind usually transfers energy to the ocean surface, creating oscillatory motion. Individual water particles generally move in roughly circular or orbital paths and return near their original positions, especially in deep water, rather than being transported forward with the wave over a long distance. Thus the wave can carry energy toward the coast even though the bulk of the water does not move forward at the same speed. Option B is correct. Permanent land and mountains are not transported by ordinary waves, and clouds are atmospheric features rather than the main substance carried by a sea wave. Breaking waves can move sediment locally, but that does not change the basic answer.

Correct answer: C. Erosion and deposition

Coastal waves are important exogenic agents because they modify the shore through erosion, transportation, and deposition. Their repeated impact can remove material from cliffs, beaches, and rocky platforms, producing erosional features such as sea caves, arches, stacks, and wave-cut platforms. At places where wave energy decreases, the carried sand and other sediment are deposited, helping form beaches, spits, bars, and related depositional features. Therefore option C gives the complete geomorphic work associated with waves. Glacier formation is controlled by persistent snow accumulation and ice processes, volcanic eruptions by internal Earth forces, and desert formation is not the exclusive work of waves. The wording in D is especially incorrect because coastal waves do both erosion and deposition.

Correct answer: D. Friction increases in shallow water

As a wave approaches the shore, the depth of the water becomes smaller. The lower part of the wave begins to interact with the seabed, and friction with the shallow bottom slows that part of the wave. The upper part continues moving faster, so the wave becomes steeper and higher. When the crest becomes unstable, it topples forward and breaks, releasing energy in the surf zone. Thus option D is the best answer. The process is not caused by a change in sky colour, a fall in salinity, or pressure decreasing in deep water. The important contrast is that bottom friction becomes increasingly influential in shallow water; in deep water, the seabed has little direct effect on ordinary surface waves.

Correct answer: A. It can form due to sudden sea-floor disturbance

A tsunami is a series of long-wavelength sea waves generated when a large volume of seawater is suddenly displaced. The displacement may result from an undersea earthquake that vertically moves the sea floor, although submarine landslides, volcanic activity, or other disturbances can also produce one. In deep water, a tsunami may travel rapidly with modest height; near the coast, decreasing depth slows the wave and can increase its height dramatically. Option A is correct because sudden sea-floor disturbance is a major cause. A tsunami is not merely a wind-generated surface wave, is not the same as a neap tide, and is not a river current. Its origin and very long wavelength distinguish it from ordinary waves.

Correct answer: A. Tide is regular rise and fall of sea level

A tide is the periodic rise and fall of the sea level over a broad coastal area, produced mainly by the gravitational attraction of the Moon and, to a lesser extent, the Sun, together with Earth-Moon motion. A normal surface wave is a shorter-scale oscillation usually generated by wind transferring energy to the water surface. Waves may also arise from other disturbances, so they are not always caused by the Moon. Therefore option A is correct: it states the simplest defining feature of a tide. Option B wrongly treats a tide as a wind wave, option C incorrectly claims every wave is lunar, and option D ignores the difference in causes, scale, and movement. Both involve moving seawater, but they are not identical phenomena.

Correct answer: B. Helping ships enter and leave

Tides periodically change the depth and water level in coastal waters. At high tide, the increased depth can allow vessels to cross shallow entrances, tidal bars, channels, or river mouths that may be difficult to navigate at low tide. Port authorities can therefore schedule the entry and departure of ships according to the predicted tidal cycle. This is especially useful for vessels with a deep draught and for ports located in estuaries or shallow coastal settings. Option B is correct. Tides do not build mountains, stop clouds, or dry the sea. The benefit is not that every port depends on tides in exactly the same way; rather, in suitable ports, the temporary rise in water level improves navigability and helps ships move safely through shallow approaches.

Correct answer: C. From regular rise and fall of tides

Tidal energy is renewable energy obtained by exploiting the predictable movement of seawater associated with tides. A tidal barrage, lagoon, or tidal-stream turbine can use the difference in water level between high and low tide, or the strong horizontal flow produced as tidal water enters and leaves an estuary or channel. The gravitational interaction of Earth, Moon, and Sun drives the cycle, but the usable energy is captured from moving water or the changing head of water, not from moonlight itself. Therefore option C is correct. Sea sand may be transported by tides but is not the energy source; cloud shadows have no such role. Tidal energy is more predictable than wind or solar generation, although suitable coastal sites and environmental planning are necessary.

Correct answer: D. Two high and two low

In many coastal areas, the usual semi-diurnal tidal pattern produces approximately two high tides and two low tides during one lunar day, which lasts about 24 hours and 50 minutes. The interval between successive high tides, or between successive low tides, is roughly 12 hours and 25 minutes, while a high tide is separated from the next low tide by about 6 hours and 12 minutes. Therefore option D is the standard general answer. Tidal patterns vary: some coasts have diurnal tides with one high and one low tide, and mixed tides may have unequal heights. However, the question asks about most coastal areas in the general school-level sense, so two high and two low tides is correct.

Correct answer: A. Tide timing changes

Tides are the periodic rise and fall of sea level produced mainly by the Moon’s gravitational attraction and partly by the Sun. As the Moon changes its position relative to Earth and the Sun, the direction and combined strength of these gravitational effects change. Consequently, the time of high and low tides shifts from day to day, and the tidal range may also vary. Therefore, option A is correct because changing lunar position affects when tides occur. The other choices are incorrect: tides do not dry the sea, ordinary waves do not disappear, and gravity does not end; it continues to act throughout the Earth–Moon system.