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The velocity of the mass as a function of time \( t \) is described by the equation:

\[
v(t) = v_{\text{max}} \cdot \sin(\omega t + \phi)
\]

where:
\begin{itemize}
    \item \( v_{\text{max}} \) is the maximum velocity (amplitude of velocity),
    \item \( \omega \) is the angular frequency of the oscillations,
    \item \( \phi \) is the initial phase.
\end{itemize}

The angular frequency \( \omega \) is related to the period \( T \) by the formula:

\[
\omega = \frac{2\pi}{T}
\]

For a period \( T = 0.4 \) seconds:

\[
\omega = \frac{2\pi}{0.4} = 5\pi \, \text{rad/s}
\]

If the mass starts moving from the equilibrium position with an upward velocity, the initial phase \( \phi \) is 0. Then, the velocity equation becomes:

\[
v(t) = 0.8 \cdot \sin(5\pi t)
\]

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