Edexcel M5 (Mechanics 5) 2017 June

1. \hspace{0pt} [In this question, \(\mathbf { i }\) and \(\mathbf { j }\) are perpendicular unit vectors in a horizontal, \(x - y\) plane.]

A bead \(P\) of mass 0.08 kg is threaded on a smooth straight horizontal wire which lies along the line with equation \(y = 2 x - 1\). The unit of length on both axes is the metre. Initially the bead is at rest at the point \(( a , b )\). A force \(( 6 \mathbf { i } - 2 \mathbf { j } ) \mathrm { N }\) acts on \(P\) and moves it along the wire so that \(P\) passes through the point \(( 5,9 )\) with speed \(10 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). Find the value of \(a\) and the value of \(b\).

2. [In this question, \(\mathbf { i }\) and \(\mathbf { j }\) are perpendicular unit vectors in a horizontal plane and \(\mathbf { k }\) is a unit vector vertically upwards.] A particle of mass 2 kg moves under the action of a constant gravitational force \(- 19.6 \mathbf { k } \mathrm {~N}\). The particle is subject to a resistive force \(- \mathbf { v }\) newtons, where \(\mathbf { v } \mathrm { m } \mathrm { s } ^ { - 1 }\) is the velocity of the particle at time \(t\) seconds.

1. By writing down an equation of motion of the particle, show that \(\mathbf { v }\) satisfies the differential equation $$\frac { \mathrm { d } \mathbf { v } } { \mathrm {~d} t } + 0.5 \mathbf { v } = - 9.8 \mathbf { k }$$ When \(t = 0 , \mathbf { v } = ( 4 \mathbf { i } - 6 \mathbf { j } + 11.6 \mathbf { k } )\)
2. Find \(\mathbf { v }\) when \(t = \ln 4\)

1. The position vectors of the points \(P\) and \(Q\) on a rigid body are \(( \mathbf { i } - 2 \mathbf { j } + 3 \mathbf { k } ) \mathrm { m }\) and \(( \mathbf { i } - \mathbf { j } + \mathbf { k } ) \mathrm { m }\) respectively, relative to a fixed origin \(O\). A force \(\mathbf { F } _ { 1 }\) of magnitude 6 N acts at \(P\) in the direction \(( \mathbf { i } - 2 \mathbf { j } + 2 \mathbf { k } )\). A force \(\mathbf { F } _ { 2 }\) of magnitude 14 N acts at \(Q\) in the direction \(( 3 \mathbf { i } - 6 \mathbf { j } + 2 \mathbf { k } )\). When a force \(\mathbf { F } _ { 3 }\) acts at \(O\), which is also a point on the rigid body, the system of three forces is equivalent to a couple of moment \(\mathbf { G }\)
   1. Find \(\mathbf { F } _ { 3 }\)
   2. Find G
   When an additional force \(\mathbf { F } _ { 4 } = ( \mathbf { i } + 3 \mathbf { j } + 4 \mathbf { k } ) \mathrm { N }\) also acts at \(O\), the system of four forces is equivalent to a single force \(\mathbf { R }\).
2. Write down \(\mathbf { R }\).
3. Find an equation of the line of action of \(\mathbf { R }\) in the form \(\mathbf { r } = \mathbf { a } + t \mathbf { b }\), where \(\mathbf { a }\) and \(\mathbf { b }\) are constant vectors and \(t\) is a parameter.

1. A uniform lamina \(P Q R\) of mass \(m\) is in the shape of an isosceles triangle, with \(P Q = P R = 5 a\) and \(Q R = 6 a\). The midpoint of \(Q R\) is \(T\).
   1. Show, using integration, that the moment of inertia of the lamina about an axis which passes through \(P\) and is parallel to \(Q R\), is \(8 m a ^ { 2 }\).
   2. Show, using integration, that the moment of inertia of the lamina about an axis which passes through \(P\) and \(T\), is \(1.5 m a ^ { 2 }\).
      [0pt] [You may assume without proof that the moment of inertia of a uniform rod, of mass \(m\) and length \(2 l\), about an axis perpendicular to the rod through its midpoint is \(\frac { 1 } { 3 } m l ^ { 2 }\) ]
      (4)
   The lamina is now free to rotate in a vertical plane about a fixed smooth horizontal axis \(A\) which passes through \(P\) and is perpendicular to the plane of the lamina. The lamina makes small oscillations about its position of stable equilibrium.
2. By writing down an equation of rotational motion for the lamina as it rotates about \(A\), find the approximate period of these small oscillations.

1. A uniform rod \(A B\), of mass \(M\) and length \(2 L\), is free to rotate in a vertical plane about a smooth fixed horizontal axis through \(A\). The rod is hanging vertically at rest, with \(B\) below \(A\), when it is struck at its midpoint by a particle of mass \(\frac { 1 } { 2 } M\). Immediately before this impact, the particle is moving with speed \(u\), in a direction which is horizontal and perpendicular to the axis. The particle is brought to rest by the impact and immediately after the impact the rod moves with angular speed \(\omega\).
   1. Show that \(\omega = \frac { 3 u } { 8 L }\)
   Immediately after the impact, the magnitude of the vertical component of the force exerted on the \(\operatorname { rod }\) at \(A\) by the axis is \(\frac { 3 M g } { 2 }\)
2. Find \(u\) in terms of \(L\) and \(g\).
3. Show that the magnitude of the horizontal component of the force exerted on the rod at \(A\) by the axis, immediately after the impact, is zero. The rod first comes to instantaneous rest after it has turned through an angle \(\alpha\).
4. Find the size of \(\alpha\). \includegraphics[max width=\textwidth, alt={}, center]{3ce3d486-0c4d-4d30-be86-e175b303fda8-19_56_58_2631_1875}

6. A small object \(P\), of mass \(m _ { 0 }\), is projected vertically upwards from the ground with speed \(U\). As \(P\) moves upwards it picks up droplets of moisture from the atmosphere. The droplets are at rest immediately before they are picked up. In a model of the motion, \(P\) is modelled as a particle, air resistance is assumed to be negligible and the acceleration due to gravity is assumed to have the constant value of \(g\). When \(P\) is at a height \(x\) above the ground, the combined mass of \(P\) and the moisture is \(m _ { 0 } ( 1 + k x )\), where \(k\) is a constant, and the speed of \(P\) is \(v\).

1. Show that, while \(P\) is moving upwards $$\frac { \mathrm { d } } { \mathrm {~d} x } \left( v ^ { 2 } \right) + \frac { 2 k v ^ { 2 } } { ( 1 + k x ) } = - 2 g$$ The general solution of this differential equation is given by \(v ^ { 2 } = \frac { A } { ( 1 + k x ) ^ { 2 } } - \frac { 2 g } { 3 k } ( 1 + k x )\),
   where \(A\) is an arbitrary constant. Given that \(U = \sqrt { 2 g h }\) and \(k = \frac { 7 } { 3 h }\)
2. find, in terms of \(h\), the height of \(P\) above the ground when \(P\) first comes to rest.