  We use an alternative method to the Bethe-Salpeter equation, the N-Quantum
approximation (NQA), for studying bound states in motion. We use this method to
find a relativistic equation for weakly bound states of two constituents with
different masses. We present rules for interpreting simple diagrams associated
with the NQA. We can use these rules to construct the bound state equations
directly, avoiding some of the complications of the process. The final result
is a bound state equation that shows Lorentz contraction in the direction of
motion explicitly. This result matches that of [3] found using the
Bethe-Salpeter equation. We briefly discuss some other applications of the NQA
in studying the effects of motion on bound states.
