Problems

Write a short account of the method of reciprocation, shewing particularly how to reciprocate a circle into a conic of any species. Give some examples shewing the power of the method. \(S\) is the focus of a given conic and a line \(l\) meets the corresponding directrix in \(Z\). \(l'\) is the line joining \(Z\) to the pole of \(l\). A second conic is drawn having \(S\) as one focus and touching \(l, l'\). A common tangent to the two conics touches them at \(Q, Q'\); shew that \(QQ'\) subtends a right angle at \(S\).

\(OX, OY\) are conjugate lines with respect to a fixed conic. \(A\) is any fixed point. A fixed circle through \(O\) and \(A\) cuts \(OX\) in \(P\), and \(AP\) meets \(OY\) in \(Q\). Shew that the locus of \(Q\) is a conic. As a particular case, shew that, if a point moves so that the line joining it to a fixed point is perpendicular to its polar with respect to a conic, the curve traced out is a rectangular hyperbola with its asymptotes parallel to the axes of the original conic. Deduce that four normals can be drawn from any point to an ellipse.

Shew how to sum the series \(a_0+a_1x+\dots+a_nx^n+\dots\), whose coefficients satisfy the relation \(a_n+pa_{n-1}+qa_{n-2}+ra_{n-3}=0\), \(p, q, r\) being given numbers. In the case where \(3a_n-7a_{n-1}+5a_{n-2}-a_{n-3}=0\), and \(a_0=1, a_1=8, a_2=17\), shew that \(2a_n=20n-7+3^{2-n}\).

Shew that

1. [(i)] \(1+2(\cos\alpha + \cos 2\alpha + \dots + \cos n\alpha) = \sin(n+\tfrac{1}{2})\alpha \csc \frac{\alpha}{2}\).
2. [(ii)] \(1+2(\cos\alpha\cos\theta+\cos 2\alpha\cos 2\theta+\dots+\cos n\alpha\cos n\theta) = \frac{\cos n\alpha \cos(n+1)\theta - \cos(n+1)\alpha \cos n\theta}{\cos\theta - \cos\alpha}\).

Deduce that \begin{align*} 1^2\cos\theta + 2^2\cos 2\theta + \dots + n^2\cos n\theta \\ = \tfrac{1}{2}\csc^2\tfrac{1}{2}\theta \{(n+1)^2\cos n\theta - n^2\cos(n+1)\theta - \sin(n+\tfrac{1}{2})\theta\csc\tfrac{1}{2}\theta\}. \end{align*}

Prove that the following definitions of the curvature of a curve at a point \(P\) lead to the same value.

1. [(i)] The rate, in radians per unit of arc, at which the tangent turns.
2. [(ii)] The limit to which the reciprocal of the radius of a circle touching the curve at \(P\) and passing through an adjacent point \(Q\) tends, as \(Q\) tends to \(P\).

Examine the nature of the evolute of a given curve in the neighbourhood of the following points on the given curve

1. [(a)] a point of inflexion,
2. [(b)] a cusp,
3. [(c)] a point of maximum or minimum curvature.

Trace the curve \(b(ay-x^2)=x^3\), and shew that the evolute has a point of inflexion corresponding to the origin of the curve.

Give some account of the theory of a framework of rods, dealing with (i) the number of rods necessary for a frame with \(n\) joints to be just rigid, (ii) the graphical determination of stresses (a) in a 'simple' frame under given forces at the joints, (\(\beta\)) in a non-simple frame. Illustrate by the two cases of a pentagon \(ABCDE\) of rods jointed at the corners, (a) with connecting rods \(AC, CE\), (b) with connecting rods \(AD, CE\), in equilibrium under three given forces at the corners \(B, C, E\).

A thin wire has the form of a circle in a vertical plane with centre \(C\). \(A, B\) are pegs attached to the wire so that \(CA, CB\) make angles \(\alpha\) on opposite sides of the downward vertical through \(C\). A small ring of mass \(M\) can slide on the wire, and is attached to two strings passed over the pegs with masses \(m\) hanging from their ends. Write down the potential energy of the system when the radius to \(M\) makes an angle \(\theta\) with the vertical. Hence discuss the stability of equilibrium positions in the cases \[ M \gtreqqless m\sin\alpha. \]

A particle of mass \(m\) is attached by a string to a point on the circumference of a fixed circular cylinder of radius \(a\) whose axis is vertical, the string being initially horizontal and tangential to the cylinder. The particle is projected with velocity \(v\) at right angles to the string along a smooth horizontal plane so that the string winds itself round the cylinder. Shew (i) that the velocity of the particle is constant, (ii) that the tension in the string is inversely proportional to the length which remains straight at any moment, (iii) that if the initial length of the string is \(l\) and the greatest tension the string can bear is \(T\), the string will break when it has turned through an angle \[ l/a-mv^2/aT. \]

Shew that if a number of particles connected by inelastic strings move under no forces, their linear momentum and energy are constant. Three equal particles \(A, B, C\) connected by inelastic strings \(AB, BC\) of length \(a\) lie at rest with the strings in a straight line on a smooth horizontal table. \(B\) is projected with velocity \(V\) at right angles to \(AB\). Shew that the particles \(A\) and \(C\) afterwards collide with relative velocity \(\frac{2V}{\sqrt{3}}\). If the coefficient of restitution is \(e\), find the velocities of the three particles when the string is again straight.