Page 56 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

48 Fundamentals of Ocean Renewable Energy

FIG. 3.1 Newton’s Law of Universal Gravitation.

FIG. 3.2 Earth-Moon system. The Earth and Moon attract each other due to Newton’s Law of
Universal Gravitation (solid arrows), but this is balanced by the outward directed centrifugal force
(dashed arrows).

the centre of gravity of the Earth-Moon system actually lies within the body
of the Earth; indeed, it is around 1700 km below the surface of the Earth.
The Earth-Moon system turns about this centre of gravity once per month,
and so every object on Earth turns with it. The centrifugal force (the dashed
arrow directed outward from the Earth on Fig. 3.2) is the same everywhere,
because all points on Earth experience the same orbital motion. However, the
gravitational attraction of the Moon is progressively weaker as we move away
from the Moon, as stated by Newton’s Law of Universal Gravitation (Eq. 3.1).
The result is that the centrifugal force and the Moon’s gravity cannot balance
each other everywhere. The slight imbalances that occur are what cause the
tides in the ocean. These imbalances are called the tide generating forces.
At the centre of the Earth (Fig. 3.3), the centrifugal force (dashed arrow)
is equal and opposite to the Moon’s gravitational pull (solid arrow). At point A
(position on the Earth’s surface closest to the Moon), the Moon’s gravity exceeds
the centrifugal force, and this results in a net force towards the Moon (block
arrow at A). At point B (position on the Earth’s surface furthest from the Moon),
the Moon’s gravity is weaker than the centrifugal force. This results in a net
force directed away from the moon at B (block arrow at B).
The tide generating forces lead to a deformation in the shape of the ocean,
stretching it out in both directions along the Earth-Moon axis (Fig. 3.4). Because