  The motion of a Dp-brane in the background of a stack of coincident
NS5-branes is analysed as the motion of a relativistic point particle in the
transverse space of the five-branes. In this system, the particle experiences a
proper acceleration orthogonal to its proper velocity due to the background
dilaton field which changes the dynamics from that of a simple geodesic motion.
In particular, we show that in the vicinity of the five-branes, it is this
acceleration which is responsible for modifying the motion of the radial mode
to that of an inverted simple harmonic oscillator leading to the tachyonic
instability.
