296
THE
RELATIVITY
PRINCIPLE
consideration is
completely
determined
by
two
variables
(V0
and
E0),
i.e.,
if the
system's
equation of
state
can
be
considered
as a
relation
between
p0,
V0
and
E0,
then, with the
help of
the
above-mentioned equations,
the
equation
of
state
can
be
brought to
the
form
v(q,p,V,E)
=
0
.
Similarly,
equation (18c)
can
be
brought
to
the
form
[76]
G
=
q
E
+
pV
ß
*
-tf-
(18d)
which,
in
combination with the
equations expressing
the principle of
conservation of
momentum
AG
x
At
S
K
,
etc.,
Jj
completely
determines the translational
motion of
the
system
as a
whole,
if
in
addition
to
the quantities
E
Kx,
etc.
one
also
knows
E,
p,
and
V
as
functions
of
time,
or
if instead
of
the last three functions
one
knows
three
equivalent
data
regarding
the conditions under
which
the
motion
of the
system
is
taking place.
[77] §14. Examples
Let
the
system
under consideration consist
of
electromagnetic
radiation,
which
is enclosed in
a
massless
hollow
body
whose
walls balance the radiation
pressure.
If
no
external forces
act
upon
the
hollow
body,
we may
apply
equations
(16a)
and
(18a)
to
the entire
system (including
the
hollow body).
We
will then
have