Edexcel M3 2019 January — Question 5 16 marks

Exam BoardEdexcel
ModuleM3 (Mechanics 3)
Year2019
SessionJanuary
Marks16
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicCentre of Mass 2
TypeComposite solid with standard shapes - calculation only
DifficultyStandard +0.8 This M3 question requires multiple centre of mass calculations: (a) involves setting up and evaluating a volume integral for a solid of revolution with algebraic manipulation to reach a specific result, and (b) requires finding centres of mass of composite shapes (hollow hemisphere bowl plus liquid hemisphere cap) and combining them using the standard formula. While the techniques are standard for M3, the multi-step nature, algebraic complexity, and need to correctly handle the composite system elevate this above average difficulty.
Spec1.08a Fundamental theorem of calculus: integration as reverse of differentiation1.08d Evaluate definite integrals: between limits6.04c Composite bodies: centre of mass6.04d Integration: for centre of mass of laminas/solids

5. \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{ae189c40-0071-4a6b-91eb-8ffebe082a04-16_492_442_237_744} \captionsetup{labelformat=empty} \caption{Figure 3}
\end{figure} The region \(R\), shown shaded in Figure 3, is bounded by the circle with centre \(O\) and radius \(r\), the line with equation \(x = \frac { 3 } { 5 } r\) and the \(x\)-axis. The region is rotated through one complete revolution about the \(x\)-axis to form a uniform solid \(S\).
  1. Use algebraic integration to show that the \(x\) coordinate of the centre of mass of \(S\) is \(\frac { 48 } { 65 } r\). \begin{figure}[h]
    \includegraphics[alt={},max width=\textwidth]{ae189c40-0071-4a6b-91eb-8ffebe082a04-16_394_643_1311_653} \captionsetup{labelformat=empty} \caption{Figure 4}
    \end{figure} A bowl is made from a uniform solid hemisphere of radius 6 cm by removing a hemisphere of radius 5 cm . Both hemispheres have the same centre \(A\) and the same axis of symmetry. The bowl is fixed with its open plane face uppermost and horizontal. Liquid is poured into the bowl. The depth of the liquid is 2 cm , as shown in Figure 4. The mass of the empty bowl is \(5 M \mathrm {~kg}\) and the mass of the liquid is \(2 M \mathrm {~kg}\).
  2. Find, to 3 significant figures, the distance from \(A\) to the centre of mass of the bowl with its liquid.

5.

\begin{figure}[h]
\begin{center}
  \includegraphics[alt={},max width=\textwidth]{ae189c40-0071-4a6b-91eb-8ffebe082a04-16_492_442_237_744}
\captionsetup{labelformat=empty}
\caption{Figure 3}
\end{center}
\end{figure}

The region $R$, shown shaded in Figure 3, is bounded by the circle with centre $O$ and radius $r$, the line with equation $x = \frac { 3 } { 5 } r$ and the $x$-axis. The region is rotated through one complete revolution about the $x$-axis to form a uniform solid $S$.
\begin{enumerate}[label=(\alph*)]
\item Use algebraic integration to show that the $x$ coordinate of the centre of mass of $S$ is $\frac { 48 } { 65 } r$.

\begin{figure}[h]
\begin{center}
  \includegraphics[alt={},max width=\textwidth]{ae189c40-0071-4a6b-91eb-8ffebe082a04-16_394_643_1311_653}
\captionsetup{labelformat=empty}
\caption{Figure 4}
\end{center}
\end{figure}

A bowl is made from a uniform solid hemisphere of radius 6 cm by removing a hemisphere of radius 5 cm . Both hemispheres have the same centre $A$ and the same axis of symmetry. The bowl is fixed with its open plane face uppermost and horizontal. Liquid is poured into the bowl. The depth of the liquid is 2 cm , as shown in Figure 4. The mass of the empty bowl is $5 M \mathrm {~kg}$ and the mass of the liquid is $2 M \mathrm {~kg}$.
\item Find, to 3 significant figures, the distance from $A$ to the centre of mass of the bowl with its liquid.
\end{enumerate}

\hfill \mbox{\textit{Edexcel M3 2019 Q5 [16]}}