Unit 3: Forces and energy

Tides — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 16 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision

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Lesson summary

Tides — HIA INTERNATIONAL notes: Describe tides, their causes in the Moon’s and Sun’s gravity, spring and neap tides, and their effects.Tide = the regular change in the depth of the ocean, which can be several metres in a day.Depth = the distance from the surface of the water to …

Tidal range
Largest range
Earth tide
Moon’s gravity
Water moves more easily
Opposite bulge

Section 1 of 7What tides are

Tide

Tide the regular change in the depth of the ocean, which can be several metres in a day.

Depth

Depth the distance from the surface of the water to the bottom.

Tidal range

Tidal range the difference in depth between high and low tide.

Largest range

16.3 m in the Bay of Fundy, Canada;
- some places such as the Caribbean and Mediterranean have very small ranges.

Earth tide

Earth tide the land also rises and falls, by about 30 cm.

Timing

Timing high tides are about 12 hours apart, low tides about 12 hours apart, and a high and the next low are about 6 hours apart.

Section 2 of 7What causes tides

Moon’s gravity

Moon’s gravity the Moon pulls on the Earth; the pull is a tidal force.

Water moves more easily

Water moves more easily ocean water is pulled more easily than land, forming a bulge towards the Moon.

Opposite bulge

Opposite bulge the side furthest from the Moon also has a high tide.

Rotation

Rotation the Earth rotates once in 24 hours, so a place passes through two high tides and two low tides each day.

Sun

Sun it also produces a tidal force, but weaker because it is further away.

Section 3 of 7Spring and neap tides

Tide	Position	Tidal range
Spring tide	Earth, Moon and Sun in line (full moon or new moon)	Largest
Neap tide	Sun and Moon at right angles to the Earth	Smallest

Spring tides

Spring tides about twice a month, once each lunar month for each alignment.

Name

Name they have nothing to do with the season spring.

Section 4 of 7Effects of tides

Harbours

Harbours some can be used only when the water is deep enough for boats.

Flooding

Flooding coastal areas flood more easily at high tide in strong winds.

Danger

Danger tidal flow can be dangerous for small boats.

Food chains

Food chains at low tide birds eat shellfish, and fish move with the tides.

Volcanoes and earthquakes

Volcanoes and earthquakes earth tides may be linked with eruptions and earthquakes.

Energy

Energy moving tidal water can generate electricity.

Section 5 of 7How ideas about tides developed

1330 BCE

330 BCE a Greek sailor observed that the depth changed twice a day and thought the Moon was the cause.

21600

1600 a German scientist suggested a magnetic force of attraction, which we now know is wrong.

31687

Newton used his ideas about gravity to calculate tidal forces, with no need for experiments.

4Acceptance

Acceptance his accurate calculations showed that gravity from the Moon and Sun causes the tides.

Section 6 of 7Key terms

Term	Meaning
Tide	The regular rise and fall of sea level.
Tidal range	The difference in depth of water between high and low tide.
Tidal force	The pull of gravity from the Moon or Sun that causes tides.
Spring tide	A tide with the largest range, when Earth, Moon and Sun are in line.
Neap tide	A tide with the smallest range.
Earth tide	The change in height of the land through the day.

Section 7 of 7Exam technique

Tides are caused by the wind — they are caused by the gravity of the Moon and the Sun.

There is only one high tide a day — most places have two high tides and two low tides.

Spring tides happen in spring — they happen when the Earth, Moon and Sun are in line.

The Sun’s tidal force is larger than the Moon’s — it is weaker because the Sun is further away.

Last updated: 12 October 2026