Problems

A uniform circular cylinder of radius \(2a\) with a groove of radius \(a\) cut in its central cross-section has mass \(M\). It rests, as shown in the diagram, on a rough plane inclined at an acute angle \(\alpha\) to the horizontal. It is supported by a light inextensible string would round the groove and attached to the cylinder at one end. The other end of the string is attached to the plane at \(Q\), the free part of the string, \(PQ,\) making an angle \(2\alpha\) with the inclined plane. The coefficient of friction at the contact between the cylinder and the plane is \(\mu.\) Show that \(\mu\geqslant\frac{1}{3}\tan\alpha.\) The string \(PQ\) is now detached from the plane and the end \(Q\) is fastened to a particle of mass \(3M\) which is placed on the plane, the position of the string remain unchanged. Given that \(\tan\alpha=\frac{1}{2}\) and that the system remains in equilibrium, find the least value of the coefficient of friction between the particle and the plane.

A smooth tube whose axis is horizontal has an elliptic cross-section in the form of the curve with parametric equations \[ x=a\cos\theta\qquad y=b\sin\theta \] where the \(x\)-axis is horizontal and the \(y\)-axis is vertically upwards. A particle moves freely under gravity on the inside of the tube in the plane of this cross-section. By first finding \(\ddot{x}\) and \(\ddot{y},\) or otherwise, show that the acceleration along the inward normal at the point with parameter \(\theta\) is \[ \frac{ab\dot{\theta}^{2}}{\sqrt{a^{2}\sin^{2}\theta+b^{2}\cos^{2}\theta}}. \] The particle is projected along the surface in the vertical cross-section plane, with speed \(2\sqrt{bg},\) from the lowest point. Given that \(2a=3b,\) show that it will leave the surface at the point with parameter \(\theta\) where \[ 5\sin^{3}\theta+12\sin\theta-8=0. \]

A smooth particle \(P_{1}\) is projected from a point \(O\) on the horizontal floor of a room with has a horizontal ceiling at a height \(h\) above the floor. The speed of projection is \(\sqrt{8gh}\) and the direction of projection makes an acute angle \(\alpha\) with the horizontal. The particle strikes the ceiling and rebounds, the impact being perfectly elastic. Show that for this to happen \(\alpha\) must be at least \(\frac{1}{6}\pi\) and that the range on the floor is then \[ 8h\cos\alpha\left(2\sin\alpha-\sqrt{4\sin^{2}\alpha-1}\right). \] Another particle \(P_{2}\) is projected from \(O\) with the same velocity as \(P_{1}\) but its impact with the ceiling is perfectly inelastic. Find the difference \(D\) between the ranges of \(P_{1}\) and \(P_{2}\) on the floor and show that, as \(\alpha\) varies, \(D\) has a maximum value when \(\alpha=\frac{1}{4}\pi.\)

The end \(O\) of a smooth light rod \(OA\) of length \(2a\) is a fixed point. The rod \(OA\) makes a fixed angle \(\sin^{-1}\frac{3}{5}\) with the downward vertical \(ON,\) but is free to rotate about \(ON.\) A particle of mass \(m\) is attached to the rod at \(A\) and a small ring \(B\) of mass \(m\) is free to slide on the rod but is joined to a spring of natural length \(a\) and modulus of elasticity \(kmg\). The vertical plane containing the rod \(OA\) rotates about \(ON\) with constant angular velocity \(\sqrt{5g/2a}\) and \(B\) is at rest relative to the rod. Show that the length of \(OB\) is \[ \frac{(10k+8)a}{10k-9}. \] Given that the reaction of the rod on the particle at \(A\) makes an angle \(\tan^{-1}\frac{13}{21}\) with the horizontal, find the value of \(k\). Find also the magnitude of the reaction between the rod and the ring \(B\).

A pack of \(2n\) (where \(n\geqslant4\)) cards consists of two each of \(n\) different sorts. If four cards are drawn from the pack without replacement show that the probability that no pairs of identical cards have been drawn is \[ \frac{4(n-2)(n-3)}{(2n-1)(2n-3)}. \] Find the probability that exactly one pair of identical cards is included in the four. If \(k\) cards are drawn without replacement and \(2 < k < 2n,\) find an expression for the probability that there are exactly \(r\) pairs of identical cards included when \(r < \frac{1}{2}k.\) For even values of \(k\) show that the probability that the drawn cards consist of \(\frac{1}{2}k\) pairs is \[ \frac{1\times3\times5\times\cdots\times(k-1)}{(2n-1)(2n-3)\cdots(2n-k+1)}. \]

The random variables \(X\) and \(Y\) take integer values \(x\) and \(y\) respectively which are restricted by \(x\geqslant1,\) \(y\geqslant1\) and \(2x+y\leqslant2a\) where \(a\) is an integer greater than 1. The joint probability is given by \[ \mathrm{P}(X=x,Y=y)=c(2x+y), \] where \(c\) is a positive constant, within this region and zero elsewhere. Obtain, in terms of \(x,c\) and \(a,\) the marginal probability \(\mathrm{P}(X=x)\) and show that \[ c=\frac{6}{a(a-1)(8a+5)}. \] Show that when \(y\) is an even number the marginal probability \(\mathrm{P}(Y=y)\) is \[ \frac{3(2a-y)(2a+2+y)}{2a(a-1)(8a+5)} \] and find the corresponding expression when \(y\) is off. Evaluate \(\mathrm{E}(Y)\) in terms of \(a\).