We are able to walk and jump and move on earth’s surface only because our cellular,
skeletal and muscular structures are strong enough to withstand the gravitational force
applied on it by the earth.  In fact the gravity that holds us to the earth (1 G) is something
that we rely on quite heavily for movement.

Without gravity our earth would not be able to retain an atmosphere, or its oceans. Gravity
enabled life to evolve the way that it did on earth and without it we may never have come
to be.  Imagine what we would look like and how our evolutionary histories would be
changed if the earth’s gravity had been slightly different (If the earth had been a little more
or less massive). In fact experiencing more than the natural G-force here on earth’s
surface can be dangerous and even deadly.

G-Force stands for gravity force, but the term is normally used to denote the force that is
applied on a body when it is accelerating at high speeds.  When accelerating a body’s
inertia (the tendency for mass to stay put when at rest) creates a force on the body.  The
effects of inertia on your body may be perceptible during the stomach lurching feelings
that you experience when you are a passenger aboard a rollercoaster taking a drop!  The
force that you would feel if you were accelerating upwards very quickly is actually very
similar to how you would feel if you were on a planet with a stronger gravitational pull than
the earth.

Many people will blackout when experiencing 3 Gs and prolonged exposure to higher Gs
is life threatening. To experience 10 Gs for a full minute is lethal.  Normally death from
excessive g-force is caused by the drainage of blood from the brain.  

NASA administers Gs to astronauts-in-testing by subjecting them to high speed travel in
specialized aircraft. The force experienced by astronauts is caused directly by the
acceleration of the aircraft. G-force can also be administered in the laboratory by placing
a human or another type of animal in a centrifuge. A centrifuge is basically a big spinning
cylindrical container. The force applied on the body in this case is due to centripetal force.

Scientists continue to experiment with protective measures to counteract the intense
forces that act on pilots in extremely high speed aircraft.  The U.S. military has developed
many protective measures like pressurized breathing systems, advanced anti-G
bodysuits and physical conditioning programs to allow fighter pilots to perform unaffected
by the high-G environments that they are so often placed in.

To find out more about Newton's theory (or law) of gravity
click here.