You can use GNU-ddrescue (https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html) for ripping. It is better than
dd, because it retries failed reads and skips unreadable parts (if disk is scratched).
Open a Terminal (with for example Ctrl+Alt+T) and install GNU ddrescue:
sudo apt-get install gddrescue
than ripp disk:
ddrescue -b 2048 -d -r 2 -v /dev/sr0 <Destination>.iso <Destination>.log
Stop ripping with Ctrl + C, because reading of scratched DVDs may take long. You can resume ripping with calling same command again.
If DVD is encrypted you have to install
libdvdcss (How can I play encrypted DVD movies?) and open DVD with a movie player like vlc before ripping (GNU ddrescue will not decrypt DVD).