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All of the radiation that passes through a circle of radius
r
and area
Ar
=π
2
,
perpendicular to the direction of propagation, is absorbed by the particle, so the force of
the radiation on the particle has magnitude
Fp
A
Pr
Rc
Pr
rr
==
=
π
π
2
2
2
2
44
.
The force is radially outward from the Sun. Notice that both the force of gravity and the
force of the radiation are inversely proportional to
R
2
. If one of these forces is larger than
the other at some distance from the Sun, then that force is larger at all distances. The two
forces depend on the particle radius
r
differently:
F
g
is proportional to
r
3
and
F
r
is
proportional to
r
2
. We expect a small radius particle to be blown away by the radiation
pressure and a large radius particle with the same density to be pulled inward toward the
Sun. The critical value for the radius is the value for which the two forces are equal.
Equating the expressions for
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This note was uploaded on 06/03/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.
 Spring '08
 Any
 Physics, Force, Mass

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