An electric dipole is placed in a uniform electric field. The resultant force on it is _______.

An electron falls through a distance of $1.5 \mathrm{cm}$ in a uniform electric field of magnitude $2\times {10}^4 {\mathrm{NC}}^{-1}$. The direction of the field is reversed keeping its magnitude unchanged and a proton falls through the same distance. Compute the time of fall in each case. (Given: $m_e=9.11\times {10}^{-31} \mathrm{kg}, m_p=1.67\times {10}^{-27} \mathrm{kg},$ $q_e=-1.6\times {10}^{-19} \mathrm{C}, q_p=1.6\times {10}^{-19} \mathrm{C}$)