6 A particle, $P$, positioned at the origin, $O$, is projected with a certain velocity along the $x$-axis. $P$ is then acted on by a single force which varies in such a way that $P$ moves backwards and forwards along the $x$-axis.

When the time after projection is $t$ seconds, the displacement of $P$ from the origin is $x \mathrm {~m}$ and its velocity is $v \mathrm {~ms} ^ { - 1 }$.

The motion of $P$ is modelled using the differential equation $\ddot { x } + \omega ^ { 2 } x = 0$, where $\omega$ rads $^ { - 1 }$ is a positive constant.
\begin{enumerate}[label=(\alph*)]
\item Write down the general solution of this differential equation.\\
$D$ is the point where $x = d$ for some positive constant, $d$. When $P$ reaches $D$ it comes to instantaneous rest.
\item Using the answer to part (a), determine expressions, in terms of $\omega$, $d$ and $t$ only, for the following quantities

\begin{itemize}
  \item $X$
  \item $v$
\item Hence show that, according to the model, $v ^ { 2 } = \omega ^ { 2 } \left( d ^ { 2 } - x ^ { 2 } \right)$.
\end{itemize}

The quantity $z$ is defined by $z = \frac { 1 } { v }$.
\item Using part (c), determine an expression for $\mathrm { Z } _ { \mathrm { m } }$, the mean value of z with respect to the displacement, as $P$ moves directly from $O$ to $D$.

One measure of the validity of the model is consideration of the value of $\mathrm { z } _ { \mathrm { m } }$. If $\mathrm { z } _ { \mathrm { m } }$ exceeds 8 then the model is considered to be valid.

The value of $d$ is measured as 0.25 to 2 significant figures. The value of $\omega$ is measured as $0.75 \pm 0.02$.
\item Determine what can be inferred about the validity of the model from the given information.
\item Find, according to the model, the least possible value of the velocity with which $P$ was initially projected. Give your answer to $\mathbf { 2 }$ significant figures.
\end{enumerate}

\hfill \mbox{\textit{OCR Further Pure Core 2 2022 Q6 [10]}}