Ответ: ma = qEa = qE/mS = at^2/2 = qEt^2/(2m)S = 3•10^(-9)•5•10^3•20^2/(2•8•10^(-3)) = 15•10^(-6)•400/(16•10^(-3)) =15•25•10^(-3) = 0,375 м.