Ocean Tides

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Q . 1  Define Ocean Tides ?

A .  Ocean tides are the rise and fall of sea level due to the gravitational forces excerted by the Moon and the Sun and the rotation of the Earth .

 

Q . 2a  Describe the difference between flood and ebb currents .

A .  A flood current is during a high tide when water moves landward .

An ebb current is during low tide when water recedes seaward .

 

Q . 2b  When are flood and ebb current weak and strong ?

A .  The strongest flood and ebb currents usually occur before or near the time of high and low tides .

The weakest currents occur between the flood and ebb current called slack tides .

 

Q . 3 Write down Newton’s Laws of Universal Gravitation.

A .  Every point mass attracts every single other point mass by a force pointing along the line joining both points. This force is basically proportional to the product of the two masses and inversely proportional to the square of the distance between them.

,

where:

  • F is the force between the masses,
  • G is the gravitational constant  = 6.674×10−11 N m2 kg−2
  • m1 is the first mass,
  • m2 is the second mass, and
  • r is the distance between the centers of the masses

 

Q . 4 Using Newton’s law of gravitational motion , which pulls the earth with the larger gravitational force , the sun or the moon .

A .   Calculation of force between earth and sun

Mass of earth (m1) = 5.972 * 1024 kg

Mass of sun (m2) = 1.989 * 1030 kg

Distance between the centre of sun and earth (r) = 149598000000 + 6371000 + 696342000 = 150300713000 m

Therefore , force =  (6.674×10−11)( 5.972 * 1024)( 1.989 * 1030)/( 150300713000)2

= 3.509 * 1022 N

Calculation of force between moon and earth

Mass of moon (m3) = 7.347 * 1022 kg

Distance between the centre of earth and moon (r) = 377739000 + 6371000 + 1738000 = 385848000 m

Therefore, force = (6.674×10−11)( 7.347 * 1022)( 5.972 * 1024)/(3858480002)

= 1.966 * 1020 N

Hence the force of attraction between sun and earth is more.

 

Q . 5 Using Newton’s Law of Universal Gravitation, explain why the sun’s gravitational attraction on the front and back of the Earth is almost the same but the moon’s gravitational attraction on the front and back of the

Earth is very different.

A.  We know that, using Newton’s laws of universal gravitation, that the force is inversely proportional to the square of distance between the objects . Also the distance between the centre of earth and sun is 150300713 km. Here adding or subtracting the radius of earth (6371 km) does make any appreciable change in the force value for the calculation of force at back and front. In case of the moon, the distance between the moon and earth is 385848 km. In this case, adding or subtracting the radius of earth in the distance do make appreciable changes in the force.

 

Q. 6. How does the moon create a bulge on the side of the Earth closest to it?

A.  The Moon does not orbit the center of the Earth. The Earth and the Moon orbit the common center of gravity which is closer to the Earth’s surface than to it’s center, on the side facing the Moon. As the Earth swings around that point, the side away from it is swung outward causing the bulge. Also, the side of the Earth farthest from the Moon gets less gravitational pull from it because of the increased distance. This is why a bulge in the side of earth occur.

 

 

 

 

Q .7. What causes a tidal bulge to form on the opposite side of the Earth from the moon?

 

A .  Water also bulges on the side opposite the moon. That is evidenced by the fact that high tides are about 12 hours apart not about 24. If you think of the Earth/Moon system as one system orbiting the sun, you will notice that even though the Earth is so much heavier the Moon is heavy enough that whole system wobbles around in its path, spinning about the center of gravity of the system. This leads to centrifugal forces on the water away from the center of the mass of the Earth/Moon system on the side away from the Moon and attraction to the Moon on the side toward the Moon.

 

 

Q . 8 How many high and low tides did Woods Hole, MA experience in the last lunar day? What type of tidal pattern did Woods Hole experience?

 

A .  For the time period of 10/12/13 (00.00 hrs) till 10/12/13(24.00hrs)

 

No. of high tides = 2

No. of low tides = 2

 

Type of tidal pattern is semi diurnal.

 

 

 

Q .  9   What are the tidal heights in Woods Hole, MA?

 

A .    Tidal heights are as follows

 

For Observed Values

 

Tidal height = (2.52-1.10) = 1.42 ft

Respective time of High tide is 02.06 hrs and low tide is 10.06 hrs

Next Tidal height = (3.12-0.82) = 2.3 ft

Respective time of high tide is 15.06hrs and low tide is 22.30 hrs .

 

For Predicted value

 

Tidal height = 1.78-0.23 = 1.55 ft

 

& = 2.32-(-0.05) = 2.37 ft

 

 

Q . 10. How do the observed and predicted data compare? What may have caused any differences between the observed and predicted data for Woods Hole, MA?

 

A .    From the data given we can say that the observed values are higher than the predicted values during the low and high tide .

The difference between the observed values and predicted values are of little significance at some and is considerable at some point. This difference occurs due to the change in air pressure and and change in temperature of air and water.

 

 

Q . 11 How many high and low tides did Mobile, AL experience in the last lunar day? What type of tidal pattern did Mobile experience?

 

 

A .   For the time period of 10/12/13 (00.00 hrs) till 10/12/13(24.00hrs)

 

No of high tide = 1

No of low tide = 1

 

It follows the wave pattern (sinusoidal curve) i.e diurnal .

 

 

Q . 12   What are the tidal heights in Mobile, AL?

 

Observed Value

 

Tidal height = (1.82-0.35) = 1.47 ft

 

Respective time of high tide is 03.00 hrs and low tide is 16.06 hrs .

 

 

 

Q . 13  How do the observed and predicted data compare? What may have caused any differences between the observed and predicted data for Mobile ?

 

A .   Here the observed and predicted value are very much in unison . Very small difference exist between them . Maximum difference between them is observed at 22.30 hrs CDT . The difference between the observed value and predicted value is due the wind speed  mainly.

 

 

 

Q . 14   How many high and low tides did Seattle, WA experience in the last lunar day? What type of tidal pattern did Mobile experience?

 

A .   No of high tide = 2

No of low tide = 2

 

Tidal pattern is mixed pattern .

 

 

 

 

Q . 15  What are the tidal heights in Seattle, WA?

 

A .   Observed value

       

Tidal heights = 10.76-(-0.13) = 10.89 ft

Respective time of high tide is 12.30 hrs and low tide is 05.00 hrs.

 

Tidal heights = 8.63-5.45 =3.18 ft

Respective time of high tide is 23.30 hrs and low tide is 18.42 hrs .

 

Predicted value  

Tidal height = 10.76-(-0.01) = 10.77ft

 

Tidal height = 8.76-6.03 = 2.73 ft

 

 

Q .  16   How do the observed and predicted data compare? What may have caused any differences between the observed and predicted data for Seattle, WA?

 

A .  Here the observed values are smaller than predicted value . Also the difference is very small and is of the order 0.01 to 0.4 roughly .

 

The difference is caused basically due to change in wind speed , air temperature and air pressure .

 

 

 

Q . 17  Use the data provided for Wrightsville Beach and Wilmington, NC to complete Table I below.

 

 

A .                                    Table I. Tidal Height for Wrightsville Beach, NC (2009)

 

Tidal Heights (ft)

  New Moon First Quarter Full Moon Second Quarter
Jan 2.9 2.5 3.6 3.8
Feb 4.0 3.1 5.7 2.3
Mar 4.0 4.0 5.1 1.6
Apr 5.2 2.8 4.3 2.8
May 4.0 3.1 , 4.0 4.8 3.0
Jun 4.7 4.1 4.5 2.4
Jul 5.9 3.7 2.8 3.2
Aug 4.6 3.6 4.0 5.4
Sep 5.2 2.0 3.7 2.4
Oct 4.5 2.0 4.4 2.8
Nov 3.6 2.8 3.7 4.1
Dec 4.8 2.8 5.7 , 2.9 3.8

 

 

Q . 18   When the moon aligns with the sun we have either a new moon, moon between Earth and sun, or a full moon, moon is behind the Earth. At this special time, the tidal effects from both the sun and moon combine. Using Table I below, describe here if your data supports this idea.

 

A .   During new moon the tidal heights are like 5.9 , 5.2 , 4.9 , 4.8 ft etc . But the tidal height during full moon in the month of January is small (2.9ft). Also during full moon the tidal heights are like 5.7 , 4.5 ,4.8 ft etc. . But here also in the month of july tidal height is small(2.8 ft) .

So data’s given in table 1 satisfies the above concept well .

 

 

Q  . 19   During spring and fall equinox, the plane of the lunar orbit matches the plane of the Earth’s orbit around the sun. During these two times of the year, the pull from the sun and moon are aligned. Use the data in Table I below and describe the high and low tides around March 21 and Sept 21.

 

A .    During spring equinox , both moon and sun lies on the same side of earth and that too in the same straight line . Hence this causes a high tide. From the table it can be seen that the tidal heights are 4.0 ft , 5.2 ft etc

During fall equinox, moon lies on the opposite side of sun. So the side closest to moon experience high tide . Due to very less percentage change in the distance between the sun and the back of earth there is hardly any influence on it due to sun’s gravitation , where as the side of earth closest to sun due to high gravitational forces experience high tide . From the table it can be seen that tidal heights are like    5.2 ft 3.7 ft etc .

 

 

Q .  20    The Earth travels around the sun in an elliptical orbit. The Earth is closest to the sun in January when it is about 91 million miles away. In July it is furthest at almost 95 million miles away. Should being closer to the sun cause stronger tides? Explain. Does your data in Table I support this?

 

A .  No it does not cause considerable stronger tides . This is due to the fact the percentage change in the distance between them is very small therefore , very little effect on tides . From table 1 we can say that the tidal heights are 2.9 ft , 3.6 ft , 3.8 ft etc . in the month of January . Though the sun and earth are closest in January, it has no effect or very little effect on the tides .

 

 

 

Q . 21                                 Table II. Tidal Schedule for Wrightsville Beach, NC (2009)

 

January

Day Time of Moon Rise Time of Moon Set Time moon highest in sky
1st 10.18 hrs 22.09 hrs 16hr13min30sec
2nd 10.44 hrs 23.08 hrs 18hr24min20sec

 

 

 

                           

                           Table III. Tidal Schedule for Wrightsville Beach, NC (2009)

 

January

Day

Low Tide Time

High Tide Time

Low Tide Time

High Tide Time

Time moon highest in sky

1st

04.06am

10.20am

04.40pm

10.50pm

04.13pm

2nd

04.53am

11.00pm

05.26pm

11.37pm

06.24pm

 

 

 

Q . 22  When the moon is highest in the sky, or overhead, should you be in the tidal bulge? Explain.

 

A .  Yes , there will be tidal bulge . This is due to the fact that tidal bulge depends on the distance between moon and the Earth . Since , distance between the moon and the earth reduces in this case as a result tidal bilge occurs .

 

 

 

Q . 23   Does your data in Table III show a high tide when the moon is directly overhead? Explain your findings.

 

A .   No ,   This is because when the moon is directly overhead does not ensure it is close to the earth .