Ocean 10, R. Lopez                                                                         West Valley College

Study Questions 11: Tides

1)      What are tides?

2)      The Earth and Moon spin together as a system in space, what is the name of the center of mass for this Earth-Moon system?

3)      What is gravitational force? The formula for gravitational force.

4)      The Sun is 27 million times more massive than the moon, but the moon has a stronger gravitational attraction the Earth’s oceans.  Why (think of Newton’s gravitational formula)?

5)      Where are gravitational forces strongest between the Earth and Moon?

6)      Where are gravitational forces weakest between the Earth and Moon?

7)      What is centrifugal force (inertia)?

8)      Are centrifugal forces on all points on Earth equal in the Earth-Moon system? 

9)      Are the centrifugal forces at different points parallel to each other or do they point toward the center of mass of the moon like gravitational forces?

10)   Where are centrifugal forces stronger than gravitational forces on Earth in the Earth-Moon system?

11)   Where are gravitational forces stronger than centrifugal forces on Earth in the Earth-Moon system?

12)   Sketch the Earth-Moon system below. Draw the gravitational forces and centrifugal forces at the points like the nadar and zenith, north and south poles, the CE (just like in lecture).  Make sure you draw the arrows in lengths proportional to their strength (i.e., gravitational force arrows are longer on the side closest to the moon).  Also, label the barycenter. CE refers to the center of mass of Earth, CM will be the center of mass of the moon.












13)   What are tractive forces?  Label the tractive forces on the figure above.

14)   On the diagram in question 12 above, label the bulge that is the result of gravitational forces and the bulge that is the result of the centrifugal forces.

15)   Recall that the bulges are the crests (high tides) of the tide waves and that the areas between the crests (low tides) are the troughs of the tide waves.  In the equilibrium theory of tides, does the Earth spin beneath these bulges or do the bulges move around the Earth?

16)   What is a lunar tide? 

17)   Recall that the Sun-Earth system also causes tide bulges due to the gravitational and centrifugal forces associated with this system.  These work just like the Earth-Moon system but at only 46%.  What are solar tides?

18)   When the Sun, Earth, and Moon are all in a line, the solar and lunar tides will be additive, resulting in higher high tides and lower low tides.  What are these tides called?

19)   When do the tides in the preceding question occur (i.e., what phase of the Moon)?

20)   When the Sun, Earth, and Moon form a right angle, the solar tide will tend to diminish the lunar tide.  What are these tides called? 

21)   When do they occur (i.e., what phase of the Moon)?

22)   What are semidiurnal tides, diurnal tides, and mixed tides (know their patterns)?

23)   What is the tide range?

24)   Why do we get semidiurnal and mixed tides? In other words, as the Earth rotates through the tidal bulges, what gets in the way of the rhythmic rise and fall of the tides to cause these semidiurnal and mixed tides?

25)   What are amphidromic points?

26)   In the northern hemisphere, when a tide crest enters an ocean basin, why does the tidal crest move in a counterclockwise motion when the Coriolis Effect must cause everything else to rotate to the right?

27)   Why does the rotation about the amphidromic point in the Pacific Ocean just west of the North American coastline cause high tide to reach Santa Cruz half-hour earlier than it reaches San Francisco?

28)   What is a tidal bore?