  We study the interaction of two D-particles in the space-time of the shock
wave. We first write the amplitude in string theory and find that, at large
distances from the shock-wave source, the O(v^4) term in the relative velocity
v is an \alpha'-independent function of the interbrane separation b. The
amplitude is therefore that of supergravity--for large b, only closed-string
massless modes contribute. We then show how the same result is obtained in the
matrix model (at small b) by setting up the formulation of the dimensionally
reduced super Yang-Mills theory in the curved background of the shock wave.
