Test of F=ma for small accelerations

We have used a torsion balance to test Newton's second law in the limit of small forces and accelerations. We were able to verify the proportionality between force and acceleration down to accelerations of $5 \times 10^{-14}\; \mbox{m}/\mbox{s}^2$. This is approximately three orders of magnitude lower than a previous measurement and provides a stringent constraint on theories involving a modification of Newtonian dynamics to explain the flatness of galactic rotation curves.