\strut - Used to create an invisible box with no width, height 8.6pt and depth 3pt.
{ \strut}
\strut command creates an invisible box with no width, height 8.6pt and depth 3pt.
\sqrt{(\ )} \sqrt{\mathstrut\rm mathstrut} \sqrt{\strut\rm strut}
\Tiny \sqrt{(\ )} \sqrt{\mathstrut\rm mathstrut} \sqrt{\strut\rm strut}
\Large \sqrt{(\ )} \sqrt{\mathstrut\rm mathstrut} \sqrt{\strut\rm strut}