  We report on results of Chandra X-ray observations of the southwestern part
of the supernova remnant (SNR) RX J1713.7$-$3946. We measure proper motions of
two X-ray bright blobs, named Blobs A and B, in regions presumably
corresponding to the forward shock of the SNR. The measured velocities are
$3800 \pm 100~\mathrm{km}~\mathrm{s}^{-1}$ and $2300 \pm
200~\mathrm{km}~\mathrm{s}^{-1}$ for Blobs A and B, respectively. Since a dense
molecular clump is located close to Blob B, its slower velocity is attributed
to shock deceleration as a result of a shock-cloud interaction. This result
provides solid evidence that the forward shock of RX J1713.7$-$3946 is indeed
colliding with dense gas discovered through radio observations reported in the
literature. The locations and velocity differences of the two blobs lead to an
estimate that the shock encountered with the dense gas $\sim 100~\mathrm{yr}$
ago. The shock velocities, together with cutoff energies of the synchrotron
X-ray spectra of the blobs, indicate that particle acceleration in these
regions is close to the Bohm limit. Blob B, in particular, is almost at the
limit, accelerating particles at the fastest possible rate. We discuss possible
influence of the shock-cloud interaction on the efficiency of particle
acceleration.
