Chapter 9 part 3Chapter 9 part 3 The Sea The Sea

Physical science 1020 C

Observations about Observations about the Seathe Sea The sea is rarely calm; it is covered The sea is rarely calm; it is covered

with waveswith waves The broadest waves travel fastestThe broadest waves travel fastest Waves seem to get steeper near Waves seem to get steeper near

shoreshore Waves break or crumble near shoreWaves break or crumble near shore Waves bend gradually toward the Waves bend gradually toward the

shoreshore

5 Questions about the 5 Questions about the SeaSea Why are there tides?Why are there tides? How do giant tides develop?How do giant tides develop? How does water in a wave move?How does water in a wave move? How is a tsunami different from How is a tsunami different from

normal waves?normal waves? Why do waves bend and break Why do waves bend and break

near shore?near shore?

Question 1Question 1

Why are there tides?Why are there tides?

The Tides The Tides (Part 1)(Part 1)

The moon’s gravity acts on the The moon’s gravity acts on the earth,earth,

but the moon’s gravity is non-but the moon’s gravity is non-uniformuniform

so the earth’s oceans are pulled so the earth’s oceans are pulled out of roundout of round

and two tidal bulges form on and two tidal bulges form on opposite sides.opposite sides.

The Tides The Tides (Part 2)(Part 2)

These bulges move as the earth These bulges move as the earth rotates, so rotates, so – each shore experiences almost two high each shore experiences almost two high

tides per daytides per day– and almost two low tides per day.and almost two low tides per day.

The heights of these tides vary with The heights of these tides vary with latitude.latitude.– They are strongest near equatorThey are strongest near equator– and weakest near poles.and weakest near poles.

The Sun’s InfluenceThe Sun’s Influence

Sun’s gravity affects Sun’s gravity affects the tidesthe tides

Strongest tides are Strongest tides are when moon and sun when moon and sun are alignedare aligned

Weakest tides are Weakest tides are when moon and sun when moon and sun are at right anglesare at right angles

Question 2Question 2

How do giant tides develop?How do giant tides develop?

Tidal ResonanceTidal Resonance

Water in a confined channel can Water in a confined channel can slosh back and forthslosh back and forth– It is at stable equilibrium when levelIt is at stable equilibrium when level– and it experiences springlike forcesand it experiences springlike forces

It’s another harmonic oscillatorIt’s another harmonic oscillator Its period depends on its inertia Its period depends on its inertia

and its stiffnessand its stiffness If the sloshing time matches the If the sloshing time matches the

tidal period, resonance occurstidal period, resonance occurs

Standing and Traveling Standing and Traveling WavesWaves

Sloshing involves Sloshing involves standingstanding waves waves– Water in a finite container has standing Water in a finite container has standing

wave modes,wave modes,– with nodes and antinodes that remain with nodes and antinodes that remain

stationary.stationary. Open water surf involves Open water surf involves travelingtraveling

waveswaves– Water in an infinite sea has traveling wave Water in an infinite sea has traveling wave

modes,modes,– with crests and troughs that move with crests and troughs that move

continuously.continuously.

Question 3Question 3

How does water in a wave move?How does water in a wave move?

Water WavesWater Waves

Sloshing involves deep water Sloshing involves deep water waves: all of the water moves waves: all of the water moves back and forthback and forth

Surface waves affect only water Surface waves affect only water near the surfacenear the surface

Water’s Motion Water’s Motion (Part 1)(Part 1)

Only the wave structure travels across the Only the wave structure travels across the water. water.

Surface water itself circles as the wave Surface water itself circles as the wave passes.passes.

The wave’s crests are formed from local The wave’s crests are formed from local water.water.

Water’s Motion Water’s Motion (Part 2)(Part 2)

The circling is strongest at the The circling is strongest at the surface,surface,

and becomes weak about 1/2 and becomes weak about 1/2 wavelength deep.wavelength deep.

Question 4Question 4

How is a tsunami different from How is a tsunami different from normal waves?normal waves?

Waves and Waves and WavelengthWavelength The longer the wavelength of The longer the wavelength of

surface wave,surface wave,– the faster it travels,the faster it travels,– the deeper it extends into the water,the deeper it extends into the water,– and the more power it conveys for its and the more power it conveys for its

amplitude.amplitude. Tsunamis areTsunamis are

– very long wavelength, deep, and very long wavelength, deep, and powerful waves.powerful waves.

– They are also not strictly surface They are also not strictly surface waves.waves.

Question 5Question 5

Why do waves bend and break Why do waves bend and break near shore?near shore?

Breaking WavesBreaking Waves Shallow water distorts a wave’s circling Shallow water distorts a wave’s circling

motion.motion. As the water grows shallower, a surface waveAs the water grows shallower, a surface wave

– slows down and its wavelength decreases.slows down and its wavelength decreases.– Its crests grow taller and more tightly bunched.Its crests grow taller and more tightly bunched.

Waves break when the water can’t form a Waves break when the water can’t form a crestcrest

The slope of the seabed affects breakingThe slope of the seabed affects breaking– If the seabed slopes gradually, there is rolling surfIf the seabed slopes gradually, there is rolling surf– If the seabed slopes sharply, plunging breakers If the seabed slopes sharply, plunging breakers

occuroccur

Changing Wave Changing Wave SpeedsSpeeds

Waves experience reflectionWaves experience reflection– Changes in wave speed cause partial Changes in wave speed cause partial

reflectionreflection– and the bigger the speed change, the more and the bigger the speed change, the more

reflectionreflection Waves experience refractionWaves experience refraction

– Changes in wave speedChanges in wave speedcan redirect the wavecan redirect the wave

– Waves bend toward shore asWaves bend toward shore asthey slow in shallowing waterthey slow in shallowing water

Summary of the SeaSummary of the Sea

The moon’s gravity causes the tidesThe moon’s gravity causes the tides The tides can cause resonant motion The tides can cause resonant motion

in channelsin channels Tidal resonances are standing wavesTidal resonances are standing waves The open sea exhibits traveling wavesThe open sea exhibits traveling waves Water moves in circles in those wavesWater moves in circles in those waves Waves break when the water gets too Waves break when the water gets too

shallow shallow