\starttext

\startformula
    \sqrt{k\over m}\quad \sqrt{\displaystyle{k\over m}}
\stopformula

\setupmathradical[sqrt][alternative=mp]

\startformula
    \sqrt{k\over m}\quad \sqrt{\displaystyle{k\over m}}
\stopformula

% watch the extra braces:

\startformula
    \sqrt{{k\over m}}\quad \sqrt{\displaystyle{k\over m}}
\stopformula

\stoptext
