Answer:
v = √ 2e (V₂-V₁) / m
Explanation:
For this exercise we can use the conservation of the energy of the electron
At the highest point. Resting on the top plate
Em₀ = U = -e V₁
At the lowest point. Just before touching the bottom plate
Emf = K + U = ½ m v² - e V₂
Energy is conserved
Em₀ = Emf
-eV₁ = ½ m v² - e V₂
v = √ 2e (V₂-V₁) / m
Where e is the charge of the electron, V₂-V₁ is the potential difference applied to the capacitor and m is the mass of the electron