2.
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\caption{Figure 2}
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\end{figure}
A uniform plank \(A B\) has weight 120 N and length 3 m . The plank rests horizontally in equilibrium on two smooth supports \(C\) and \(D\), where \(A C = 1 \mathrm {~m}\) and \(C D = x \mathrm {~m}\), as shown in Figure 2. The reaction of the support on the plank at \(D\) has magnitude 80 N . Modelling the plank as a rod,
- show that \(x = 0.75\)
A rock is now placed at \(B\) and the plank is on the point of tilting about \(D\). Modelling the rock as a particle, find
- the weight of the rock,
- the magnitude of the reaction of the support on the plank at \(D\).
- State how you have used the model of the rock as a particle.