Transcript Tides - National Schools` Observatory

Tides Simulation
The Project
 Students are presented with an
interactive simulation of the tides.
 The cause of tides are discussed
including the effect on the tides varying
alignment of the Earth with the Sun and
the Moon.
 Students are asked to take
measurements of the tidal variation
generated from the simulation, which
are then analysed and discussed.
What are Tides?
• In costal areas it can be observed that the
sea retreats from the coast before
returning at different times throughout
the day.
• This is caused by tides.
• Tides are the vertical rise and fall of the
Earth’s ocean surface, arising primarily
from the gravitational interaction of the
Sun, the Earth and the Moon.
Gravity
 Gravity is the weakest of the four fundamental
forces.
 Gravity is responsible for the attraction of
massive bodies.
 Gravitational force is always attractive and acts
along the line joining the centres of mass.
 Gravitational force is responsible for the
formation of planets, stars and galaxies.
 Gravity decides the orbital paths of the planets
and moons in our solar system, and the tidal
forces that we experience on Earth.
National Schools’ Observatory
Newton’s Law of Universal Gravitation
 The gravitational force between the two
masses m1 and m2 is described by
Newton’s Law of Universal Gravitation.
 G is the universal gravitation constant.
 This relationship is an example of an
inverse square law force.
 Forces are equal in size but in opposite
directions.
F1  F 2  G
G  6 . 67  10
National Schools’ Observatory
m1  m 2
 11
r
2
2
Nm / kg
2
What causes tides? (1)
• The gravitational field of the Moon causes the Earth’s oceans to bulge. It is the
Earth rotating through these ‘tidal bulges’, which causes the tides.
• For the majority of locations, two high tides and two low tides are experienced
every day.
•
This is due to the presence of a tidal bulge in the Earth’s oceans, which
appears on both sides of the planet.
Note : The tidal bulge in
this image is exaggerated
for demonstration purposes
What causes tides? (2)
• The gravitational force is not equal over the whole of the planet. This is
called a ‘differential gravitational force’.
• Since gravitational force falls away as 1/r2, the force on the near side of
the Earth is greater than the force on the far side. i.e. a unit mass on the
far side of the Earth feels less of a gravitational pull than a unit mass on
the near side.
• This is shown in the image below as force vectors.
What causes Tides (3)
• The tidal bulge which points away from the Moon arises from this imbalance in
gravitational force.
• If the forces on the near and far edges of the planet are compared with respect to the force
in the middle, we are left with resultant forces pointing outward from the Earth’s surface
on both sides of the planet.
• i.e. when the force vectors of the centre are subtracted from the force vectors at either side.
The result is forces which, point out from the centre of the Earth.
Tidal Force
• It helps to think of the Earth in simplified terms
as a sphere with two layers.
• The outer layer being the oceans, and the inner
layer being the solid surface, or the ocean beds.
• If we subtract the force due to gravity on the
inner sphere from the force due to gravity on the
outer sphere, we have a differential gravitational
force.
• This differential gravitational force adds a stress
to the oceans.
• The oceans, being made of water, can deform to
counter-act this gravitational stress, causing the
tides.
Tidal Lag
•
The tidal bulges do not line up
perfectly, with the line of the
Moon.
• This is due to the rotation of the
Earth being much faster than the
orbit of the Moon.
• Frictional forces between the
Earth’s solid surface and the
water, cause the tidal bulge to be
forced ahead of the Moon.
Note: The above angle is exaggerated for
demonstration purposes.
Spring and Neap Tides
• The Earth also lies in the gravitational
field of the Sun. The Sun therefore also
has an effect on the tides.
• When the Earth, Sun and Moon are in a
line (Syzygy), the Sun’s gravity adds to
the tidal force increasing the tidal
bulges.
• These are called Spring tides.
• When the Moon is at right angle’s to the
line of the Sun and the Moon
(Quadrature), the solar gravity reduces
the Moon’s contribution to the tidal
bulge.
• These are called Neap Tides.
Prediction
 Which body has the most gravitational
pull on the Earth. The Sun or the Moon?
 Which body has the most effect on the
tides. The Sun or the Moon.
 Predict when would you expect the
largest tides?
 Predict when would you expect the
smallest tides?
Setting Up the Experiment
•
Load the ‘Tides Simulator’.
•
The
symbol, will zoom out from the
Earth, displaying a ‘not to scale’ interaction
of the Sun, the Earth and the Moon.
•
The
symbol will return to the close up
view of the Earth and the Moon.
•
The symbol, will allow students to
toggle on the ‘day number’, increase or
slow down the speed of the Earth’s rotation
(close mode only), remove the influence of
the Sun and also toggle on the tide height
simulation.
Measuring and Recording
•
Provide students with a print out of the
tides worksheet.
• Ask the students, with the aid of the tides
simulation, to sketch the tides on the
Earth, which correspond to the position
of the Moon and the Sun.
• Ask the students to label, the boxes in
which a Spring tide or a Neap tide is
present.
Measuring and Recording
•
In the simulation, activate the Day
Number and Tidal Variation Option.
• The white marker attached to the Earth,
represents an observer on a boat.
• The window to the left simulates the
change in sea level due to the tides for
that observer.
• Record the sea level at high and low tide,
for each day until the day Moon has
completed a full orbit of the Earth.
Example Chart (Day Vs Tide Height)
• Ask the students to plot Tide Height Vs Days.
• Making sure to only chose one high and low tide a day.
0.8
0.6
Tide Height (m)
0.4
0.2
0
1
6
11
16
-0.2
-0.4
-0.6
-0.8
Days
21
26
Prediction Compared to Results
• Does the height of the high and low tide
vary?
• When do the tides with most variation
occur?
• What alignment of the Sun, Moon and
Earth generate the highest tides?
Discussion
 How many high and low tides are there
in one day?
 How many days are there between
periods of low tidal activity and high
tidal activity?
 How long does it take the Moon to orbit
the Earth?
Questions, Exercises and Tasks
 Tidal variation at costal areas can be much
higher than out to sea. Why is this?
 Does Jupiter have an effect on the tides?
 Why is it important to be able to predict the
tides?
 If the Moon was further away, how would
this affect the tides?
 Are there tides on the Moon?