For objects at large distances, like the Moon-Earth system
in the gravitational field of the Sun, the effects of gravitation
do not simply cancel.
When the relative position vector
of the Moon-Earth system points towards,
or away from the Sun,
then the effect of the gravitational field of the Sun pulls
the Moon and the Earth away from each other.
But, when the relative position vector
of the Moon-Earth system is perpendicular
to the line which connects the Sun with the center-of-mass of
the Moon-Earth system,
then the effect of the gravitational field of the Sun pushes
the Moon and the Earth towards each other.
These are called tidal forces since they
cause the
tides
of the ocean caused by the effects of the
gravitational field of the Moon (and the Sun) on the Earth-Ocean system.
|
|