``Reference directions'' (
) on the voltage source and
circuit elements may be chosen arbitrarily--just keep track and
be consistent
When
is positive, ``current'' is pushed from its
to its
terminal, i.e.,
will be positive if the rest of the circuit is just a wire or a resistor
The ``force drop'' across the mass
is positive when
increases in the direction
going from its
to
terminal. This can be interpreted as
the inertial reaction force of the mass that opposed the
external applied force (Newton's first law of motion)