**(a)**At what resonance ([itex]\omega = \omega_0[/itex]), what is the value of the phase angle [itex]\phi[/itex]?

**(b)**What, then, is the displacement at a time when the driving force [itex]F_{ext}[/itex] is a maximum, and at a time when [itex]F_{ext} = 0[/itex]?

**(c)**What is the phase difference (in degrees) between the driving force and the displacement in this case?

Equations related to this problem:

[tex]F_{ext} = F_0\cos{\omega t}[/tex]

[tex]x = A_0\sin{(\omega t + \phi_0)}[/tex]

[tex]\phi_0 = \tan^{-1}\frac{\omega_0^2 - \omega^2}{\omega(b/m)}[/tex]

My Answers:

**(a)**Since [itex]\omega = \omega_0[/itex], [itex]\phi_0 = \tan^{-1}0[/itex] which means [itex]\phi_0 = k\pi[/itex] for some non-negative integer k.

**(b)**[itex]F_{ext}[/itex] has its maximum value when [itex]\omega t = 2j\pi[/itex] for some non-negative integer j. The displacement is then [itex]x = A_0\sin{(2j\pi + k\pi)} = 0[/itex]. [itex]F_{ext} = 0[/itex] implies that [itex]\omega t = l\pi/2[/itex] where l is some odd positive integer.The displacement is then [itex]x = A_0\sin{(l\pi/2 + k\pi)}[/itex], so x = A

_{0}or -A

_{0}.

**(c)**This question I don't understand well. I'm guess the difference is [itex]\pi/2 + \phi_0[/itex] because the driving force is a cosine function and the displacement is a sine function with a phase angle.

Is this right?