The → gravitational force exerted on an extended body
as a result of the difference in the strength of gravity between near and far
parts of the body. The ocean tides on Earth result from the varying gravitational force of the
Moon exerted on the Earth’s oceans closest and farthest from the Moon.
Tidal force, which is the → gradient of the
gravitational force, varies as 1/r3. More specifically,
Ftidal = dF/dr = (2GMm)/r3,
where M is mass of the → primary body,
m is mass of the → secondary body,
r is distance between objects, and G the
→ gravitational constant. The total tidal force experienced
across a body is equal to the tidal force (force per unit distance) multiplied
by the diameter of that body:
Ftt = Ftidal x 2R (provided that radius
R is much smaller than r). It is obvious that the tidal force experienced
by Earth at Moon’s → perigee is larger than
that at the → apogee.
If the tidal force is stronger than a body’s cohesiveness, the body
will be disrupted. The minimum distance that a secondary comes to a
primary before it is shattered by tidal force is called its
→ Roche limit. Tidal forces create
→ tidal heating.
See also: → tidal; → force.
