206
MOTION
OF CENTER
OF GRAVITY
_
Z
(y^
+
I
%
+
PedT
xpedr
where, according to
the
energy
principle, the value of the denominator
on
the
right-hand
side
is
independent
of
time.1
Hence
we
may
write
equations
(2b)
also in the form
(2c)
de/dt
=
const.
Thus,
if
one
ascribes the inertial
mass
E/V2
to
any energy
E,
then-at least in first
approximation-the principle
of conservation
of
the
motion
of
the
center
of
gravity also
holds for
systems
in
which
electro-
magnetic processes
take place.
The
present investigation
shows
that
one
either
has
to give
up
the
fundamental
law
of mechanics,
according
to which
a body
originally
at rest
cannot perform
translational
motion
unless acted
upon by
external
forces,
or
one
has to
assume
that
a
body's
inertia
depends
on
its
energy
content
according
to
the
law
stated.
Bern,
May
1906.
(Received
on
17
May
1906)
1According
to the
interpretation
developed
in this
paper,
the principle
of
the
constancy
of
mass
is
a
special
case
of the
energy
principle.