Find the first critical speed in rpm for the aluminum shaft shown below. $E=10x{10}^6.?$ You may use the beam deflection formula given below

${}_1=\frac{\text{P}\text{a}\text{b}}{6EI},{}_2=\frac{\text{P}\text{a}\text{b}}{3EI},{}_3=\frac{Pb}{6EI}(2l+b)$

For $O$