`Real' Gravity!!!

    Many people use the manual's suggestion of `SFOR f=9.81' for
simulating gravity close to the Earth's surface. Well, I'm sorry to say
(as some others have noticed) that this is *NOT* gravity as we know it
here on terra firma. The gravitational field we experience everyday is
not a constant force as the above formula suggests.
    A constant directed force is best described by picturing two objects
in 0-gravity (i.e. outer space). Let's say you have a fountain pen and
the Space Shuttle. Now imagine strapping a 1 Newton model rocket engine
to both. If you ignite both rockets, what do you think will happen?
    Well, common sense tells you that the fountain pen is going to
accelerate very quickly. However, it could take a very long time before
you would see any noticable movement in the shuttle (assuming the rocket
had enough fuel). Why is this?
    Inertia.
    Inertia is an objects tendency to remain at rest, if it is
motionless, or to remain in motion if it is already moving. The greater
an object's mass, the greater this tendency. This is why it would take
so long for the tiny rocket's force to accelerate the shuttle. It is a
very massive object compared to the fountain pen.
    Likewise, if you create 2 spheres in Real 3D, one of them with a
small mass (10 Kg), the other with a large mass (10000 Kg), and apply a
DIRECTED FORCE downward of f=9.81, the sphere with the small mass will
accelerate much faster than the heavy sphere.
    This is fine for doing wind and stuff like that, but as Sir Isaac
Newton discovered gravity is not a constant force. It varies according
to the mass of affected objects to accelerate ALL objects at the same
rate. So how do we create such a force?
    Well, luckily Newton already came up with the formula for us: F=m*a.
Since we know that the acceleration of gravity is 9.81 m/s/s, we can
substitute this value for a. And if you look at the variables for
DIRECTED FORCE on page REFERENCE 2.16, you see that `m1' is the mass of
the object the force is targeting.
    So our new formula for gravity is: f=m1*9.81.
    No matter how heavy an object is, this formula will create a force
that is strong enough to accelerate the object at exactly 9.81 m/s/s.
    To use this formula, simply add the following tag to your DIRECTED
FORCE method (note the lowercase letters):

    SFOR f=m1*9.81

Hope that helps!

Mark Aydelotte