The time it takes for a point to reach 0 displacement from maximum displacement is [tex]\frac{\pi}{2}[/tex] of a cycle. Thus, one period ([tex]2\pi[/tex]) will take [tex]T=0.17*4=0.68s[/tex].
The frequency is simply [tex]f=\frac{1}{T}=\frac{1}{0.68}=1.47Hz[/tex].
The speed is given by: [tex]v=f\lambda=1.47*1.4 = 2.06m/s[/tex]
