Gravity and acceleration

Most effects of gravity vanish in
free fall, but effects that seem
the same as those of gravity
can be produced by an
accelerated frame of reference.
An observer in a closed room
cannot tell which of the following
is true:
Objects are falling to the floor
because the room is resting on
the surface of the Earth and the
objects are being pulled down by
gravity.
Objects are falling to the floor
because the room is aboard a
rocket in space, which is
accelerating at 9.81 m/s2 and is
far from any source of gravity.
The objects are being pulled
towards the floor by the same
"inertial force" that presses the
driver of an accelerating car into
the back of his seat.
Conversely, any effect observed
in an accelerated reference
frame should also be observed in
a gravitational field of
corresponding strength. This
principle allowed Einstein to
predict several novel effects of
gravity in 1907, as explained in
the next section.
An observer in an accelerated
reference frame must introduce
what physicists call fictitious
forces to account for the
acceleration experienced by
himself and objects around him.
One example, the force pressing
the driver of an accelerating car
into his or her seat, has already
been mentioned; another is the
force you can feel pulling your
arms up and out if you attempt
to spin around like a top.
Einstein's master insight was
that the constant, familiar pull
of the Earth's gravitational field
is fundamentally the same as
these fictitious forces.[5] The
apparent magnitude of the
fictitious forces always appears
to be proportional to the mass
of any object on which they act -
for instance, the driver's seat
exerts just enough force to
accelerate the driver at the
same rate as the car. By
analogy, Einstein proposed that
an object in a gravitational field
should feel a gravitational force
proportional to its mass, as
embodied in Newton's law of
gravitation.[6]