OCR M3 2008 January — Question 2 9 marks

Exam BoardOCR
ModuleM3 (Mechanics 3)
Year2008
SessionJanuary
Marks9
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicMoments
TypeRod hinged to wall with string support
DifficultyStandard +0.8 This is a two-rod statics problem requiring moment equations about multiple points, resolution of forces at a hinge, and systematic application of equilibrium conditions across connected bodies. While the geometry is given (avoiding trigonometric complexity), students must carefully track forces at joint B and apply moments correctly to both rods, which is more demanding than single-body equilibrium problems typical at this level.
Spec3.04a Calculate moments: about a point3.04b Equilibrium: zero resultant moment and force

2 \includegraphics[max width=\textwidth, alt={}, center]{7e0f600a-18f1-458b-8549-27fca592b19c-2_515_1065_861_541} Two uniform rods \(A B\) and \(B C\), each of length 2 m , are freely jointed at \(B\). The weights of the rods are \(W \mathrm {~N}\) and 50 N respectively. The end \(A\) of \(A B\) is hinged at a fixed point. The rods \(A B\) and \(B C\) make angles \(\tan ^ { - 1 } \left( \frac { 3 } { 4 } \right)\) and \(\beta\) respectively with the downward vertical, and are held in equilibrium in a vertical plane by a horizontal force of magnitude 75 N acting at \(C\) (see diagram).
  1. By taking moments about \(B\) for \(B C\), show that \(\tan \beta = 3\).
  2. Write down the horizontal and vertical components of the force acting on \(A B\) at \(B\).
  3. Find the value of \(W\).

2\\
\includegraphics[max width=\textwidth, alt={}, center]{7e0f600a-18f1-458b-8549-27fca592b19c-2_515_1065_861_541}

Two uniform rods $A B$ and $B C$, each of length 2 m , are freely jointed at $B$. The weights of the rods are $W \mathrm {~N}$ and 50 N respectively. The end $A$ of $A B$ is hinged at a fixed point. The rods $A B$ and $B C$ make angles $\tan ^ { - 1 } \left( \frac { 3 } { 4 } \right)$ and $\beta$ respectively with the downward vertical, and are held in equilibrium in a vertical plane by a horizontal force of magnitude 75 N acting at $C$ (see diagram).\\
(i) By taking moments about $B$ for $B C$, show that $\tan \beta = 3$.\\
(ii) Write down the horizontal and vertical components of the force acting on $A B$ at $B$.\\
(iii) Find the value of $W$.

\hfill \mbox{\textit{OCR M3 2008 Q2 [9]}}