Exercise 5: Tides.

Imagine a pair of small rectangular fish tanks (ocean basins) on a table (earth). The table is being moved back and forth (the tide generating force). Each fish tank has its own period at which it comes into resonance with the movement of the table. The resonance period is determined by the dimensions (width, length, depth) of the tank (just as the pitch of the tone produced by an organ pipe is determined by the pipe dimensions).

For a given table movement, the water will therefore slosh back and forth in both fish tanks, but more in one than in the other. The tank with the more vigorous water movement will be the one with a resonance period closer to the period of the table movement.

All ocean basins of the world ocean have dimensions that places them well away from resonance at the tidal period. Tides in the open ocean are therefore small; they do not reach amplitudes much larger than 0.5 m, but they are larger in some basins than in others.

The earth is of course not rocking back and forth like the table in our experiment; it is rotating. So imagine now that, instead of rocking the table back and forth, we expose it to gentle circular motion. What do you think will happen to the water in the fish tanks? The answer is somewhat easier to see if we replace the tanks by fish bowls.

The water in the bowls ...