CAIE M2 (Mechanics 2) 2015 November

2
\includegraphics[max width=\textwidth, alt={}, center]{e39fcb6b-7bd2-4dd5-bbd6-5de6e13b30b3-2_476_716_486_715} A uniform rigid rod \(A B\) of length 1.2 m and weight 8 N has a particle of weight 2 N attached at the end \(B\). The end \(A\) of the rod is freely hinged to a fixed point. One end of a light elastic string of natural length 0.8 m and modulus of elasticity 20 N is attached to the hinge. The string passes over a small smooth pulley \(P\) fixed 0.8 m vertically above the hinge. The other end of the string is attached to a small light smooth ring \(R\) which can slide on the rod. The system is in equilibrium with the rod inclined at an angle \(\theta ^ { \circ }\) to the vertical (see diagram).

1. Show that the tension in the string is \(20 \sin \theta \mathrm {~N}\).
2. Explain why the part of the string attached to the ring is perpendicular to the rod.
3. Find \(\theta\).

3 A particle \(P\) of mass 0.3 kg moves in a straight line on a smooth horizontal surface. \(P\) passes through a fixed point \(O\) of the line with velocity \(8 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). A force of magnitude \(2 x \mathrm {~N}\) acts on \(P\) in the direction \(P O\), where \(x \mathrm {~m}\) is the displacement of \(P\) from \(O\).

1. Show that \(v \frac { \mathrm {~d} v } { \mathrm {~d} x } = k x\) and state the value of the constant \(k\).
2. Find the value of \(x\) at the instant when \(P\) comes to instantaneous rest.
   \includegraphics[max width=\textwidth, alt={}, center]{e39fcb6b-7bd2-4dd5-bbd6-5de6e13b30b3-3_921_672_258_733} One end of a light inextensible string is attached to a fixed point \(A\). The string passes through a smooth bead \(B\) of mass 0.3 kg and the other end of the string is attached to a fixed point \(C\) vertically below \(A\). The bead \(B\) moves with constant speed in a horizontal circle of radius 0.6 m which has its centre between \(A\) and \(C\). The string makes an angle of \(30 ^ { \circ }\) with the vertical at \(A\) and an angle of \(45 ^ { \circ }\) with the vertical at \(C\) (see diagram).
3. Calculate the speed of \(B\). The lower end of the string is detached from \(C\), and \(B\) is now attached to this end of the string. The other end of the string remains attached to \(A\). The bead is set in motion so that it moves with angular speed \(3 \mathrm { rad } \mathrm { s } ^ { - 1 }\) in a horizontal circle which has its centre vertically below \(A\).
4. Calculate the tension in the string.

6
\includegraphics[max width=\textwidth, alt={}, center]{e39fcb6b-7bd2-4dd5-bbd6-5de6e13b30b3-4_647_641_260_751} A uniform circular disc has centre \(O\) and radius 1.2 m . The centre of the disc is at the origin of \(x\)-and \(y\)-axes. Two circular holes with centres at \(A\) and \(B\) are made in the disc (see diagram). The point \(A\) is on the negative \(x\)-axis with \(O A = 0.5 \mathrm {~m}\). The point \(B\) is on the negative \(y\)-axis with \(O B = 0.7 \mathrm {~m}\). The hole with centre \(A\) has radius 0.3 m and the hole with centre \(B\) has radius 0.4 m . Find the distance of the centre of mass of the object from

1. the \(x\)-axis,
2. the \(y\)-axis. The object can rotate freely in a vertical plane about a horizontal axis through \(O\).
3. Calculate the angle which \(O A\) makes with the vertical when the object rests in equilibrium.