motion in vertical direction
When entering the electric field of the capacitor the force of this field accelerates the electrons in vertical direction So you can use the following equations: $$\begin{equation}y(t)=\frac{1}{2}a_y\cdot t^2\end{equation}$$ $$\begin{equation}v_y(t)=a_y\cdot t\end{equation}$$ $$\begin{equation}a_y=-\frac {E\cdot e}{m_e}=-\frac {V_{\text p}\cdot e}{\text{d}\cdot m_e}\end{equation}$$ Typical magnitude: $$a_y=-\frac {1000~\text{V}\cdot 1{,}6\cdot 10^{-19}~\text{C}}{0{,}05~\text{m}\cdot 9{,}1\cdot 10^{-31}~\text{kg}}\approx -3{,}5\cdot 10^{15}~\frac{\text m}{\text{s}^2}$$
$E=$ electric field,
$e=$ Elementary charge,
$m_e=$ Mass of an electron,
$V_{\text p}=$ potential between capacitor plates,
d = distance between capacitor plates.