Tides

Question 1

Tides time and & amplitude

Answer 1

The time and & amplitude of high tide varies from place-to-place and from day-to-day
largest when new moon or full moon

Question 2

Tides

Answer 2

• Long period, shallow water waves
• driver (disturbing force) = gravity from sun & moon
• extremely predictable
• longest of all waves, wavelength = 1/2 Earth’s circumference
• shallow water wave

Question 3

Importance of the tides

Answer 3

• Intertidal zone
• sea level rise and sunny day flooding (flooding from sea level rise happens during the highest high tides)
• tidal energy generation (tidal energy generation can harness both potential and kinetic energy from the tides)
• ocean currents & mixing in coastal regions
• marine hazards
• surfing & fun (severn bore)
• cultural & spiritual connections to the ocean (mont saint-michel)

Question 4

What causes the tides?

Answer 4

Equilibrium theory of tides
- simplified model of tidal forcing
- originally dveeloped by Newton (late 1600s)
- considers gravitational pull between earth, moon and sun
- considered rotational effects of earth and moon around their common centers of mass
- explains why: there are two high tides per day, one high tide can be bigger than the other, there are spring and neap tides

Question 5

Equlibrium theory assumptions

Answer 5

• earth is a sphere covered with water to a unifrom depth,no continents
• ignore the effects of friction
• instantaneous equilibrium (ocean can move into place to become in equilibrium with gravity field)
• to start, just consider effect of moon (ignore the sun)

Question 6

The earth-moon system

Answer 6

• moon does not revolve around the Earth’s center
• it revolves around the earth-moon center of mass (barycenter)

Question 7

Newtons law of gravitation

Answer 7

Fg = Gm1m2 / r^2
Fg = gravitational force
m = mass
r = distance between the objects

If mass increases, gravitational force increases
If distance increases, then gravitational force greatly increases

Question 8

Formation of two tidal bulges

Question 9

Centrifugal force

Answer 9

an ‘apparent’ outward force that an object in orbit ‘feels’

Question 10

Centripetal force

Answer 10

a real force that keeps an object in orbit (gravity provides the force for earth/moon orbit)

Question 11

Gravitational force from moon

Answer 11

Towards the moon everywhere, dercreases with distance from

Question 12

Resultant force

Answer 12

Mismatch between centripetal force and gravitation force

Question 13

Tide-generating force is the horizontal component of the resultant force

Answer 13

Water piles up

Question 14

Result

Answer 14

• Tidal bulges stay aligned with Earth-moon axis,
• Earth rotates beneath tidal bulges, producing cycle of high and low tides
• semi-diurnal tide (high tide every 12.4 hours)

Question 15

Why not exactly 12 hours?

Answer 15

• lunar day is 24 hr and 50 min (longer than solar day)
• high tides are every 12.4 hrs (12 hr, 25 min) = 2 bulges lunar day

Question 16

Why do the two daily high tides have different amplitudes in some places?

Answer 16

Declamation to the Earth relative to the plane of the equator

Question 17

Diurnal vs semidiurnal tides

Question 18

Why does the tidal amplitude change with phase of the moon?

Answer 18

• the sun’s effect: sun’s gravity also attracts ocean, but has smaller effect
• 28 day tidal cycle
• spring tides and neap tides