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*Gravity Fields*

The gravitational force **inside** a **hollow
Earth** is zero.

But our Earth is **not hollow.** If the Earth were of
**uniform density**, then the force would vary linearly inside
the Earth. It is zero at the center -- just from symmetry -- and
reaches the value of the usual gravitational force at Earth's
surface at Earth's surface! This linear force looks just like the
linear force exerted by an ordinary spring. Indeed, this
oscillatory motion is exactly like the motion of a mass attached
to a spring.

**This means the force
***inside* the Earth is proportional the distance from the
center -- just like the force of a spring is proportional to how
far the spring has been stretched or compressed from equilibrium.
A mass acted on by a spring moves back and forth is simple
harmonic motion which we will study a little later. Since the
force of gravity *inside* the Earth has the same form --
linearly proportional to a distance -- it will cause the same kind
of simple harmonic motion.
**Therefore, . . .**

**Back to " Gravity" Page**
(C) 2003, Doug Davis; all rights reserved