Consider 3rd orbit of He⁺ (Helium), using non-relativistic approach, the speed of electron in
this orbit will be (take, K = 9 x 10⁹ constant, Z = 2 and Planck's constant, h = 6.6 X 10^-34 J-s) [CBSE AIPMT 2015]

Energy of electron in the 3rd orbit of He⁺ is

$E_3=-13.6\times \frac{Z^2}{n^2}\mathrm{eV}=-13.6\times \frac{4}{3^2}\mathrm{eV}$

$=-13.6\times \frac{4}{9}\times 1.6\times {10}^{-19} \mathrm{J}$

From Bohr's model, $E_3=-{\mathrm{KE}}_3=-\frac{1}{2}m{v}^2$

$\Rightarrow =-13.6\times \frac{4}{9}\times 1.6\times {10}^{-19}=-\frac{1}{2}\times 9.1\times {10}^{-31}\times v^2$

$\Rightarrow v^2=\frac{13.6\times 4\times 1.6\times {10}^{-19}\times 2}{9.1\times {10}^{-31}\times 9}$

$\Rightarrow v^2=212\times {10}^{12}\text{ or }v=1.46\times {10}^6{ \mathrm{ms}}^{-1}$