  Berry and Balazs showed that an initial Airy packet Ai(b x) under time
evolution is nonspreading in free space and also in a homogeneous time-varying
linear potential V(x,t)=-F(t) x. We find both results can be derived from the
time evolution operator U(t). We show that U(t) can be decomposed into ordered
product of operators and is essentially a shift operator in x; hence, Airy
packets evolve without distortion. By writing the Hamiltonian H as H=H_b+H_i,
where H_b is the Hamiltonian such that Ai(b x) is its eigenfunction. Then, H_i
is shown to be as an interacting Hamiltonian that causes the Airy packet into
an accelerated motion of which the acceleration a=(-H_i/( x))/m. Nonspreading
Airy packet then acts as a classical particle of mass m, and the motion of it
can be described classically by H_i.
