Edexcel M3 (Mechanics 3)

1. Prove that the centre of mass of a uniform solid hemisphere of radius \(r\) is at a distance \(\frac{3r}{8}\) from its plane face. [7 marks]

A solid cylinder of radius \(\frac{3r}{4}\) and height \(kr\), where \(k < 1\), is welded to a uniform hemisphere of radius \(r\) made of the same material, so that their axes of symmetry coincide. The figure shows the cross section of the resulting solid. If the centre of mass of this solid is at \(O\), the centre of the plane face of the hemisphere, \includegraphics{figure_5}

1. find the value of \(k\). [4 marks]

The gravitational attraction \(F\) N between two point masses \(m_1\) kg and \(m_2\) kg at a distance \(x\) m apart is given by \(F = \frac{km_1m_2}{x^2}\), where \(k\) is a constant. Given that a small body of mass \(1\) kg experiences a force of \(g\) N at the surface of the Earth, which has radius \(R\) m and mass \(M\) kg,

1. show that \(k = \frac{gR^2}{M}\). [2 marks]

A small communications satellite of mass \(m\) kg is put into a circular orbit of radius \(r\) m around the Earth. Modelling the Earth as a particle of mass \(M\) kg, and using the value of \(k\) from (a),

1. prove that the period of rotation, \(T\) s, of the satellite is given by \(T = \frac{2\pi}{R}\sqrt{\frac{r^3}{g}}\). [4 marks]

To cover transmission to any point on the Earth, three small satellites \(X\), \(Y\) and \(Z\), each of mass \(m\) kg, are placed in a common circular orbit of radius \(r\) and form an equilateral triangle as shown. \includegraphics{figure_6}

1. Show on a copy of the diagram the direction of the three forces acting on \(X\). [1 mark]
2. State, with a reason, the direction of the resultant force on \(X\). [2 marks]
3. Show that the period of rotation of \(X\) is given by \(T\sqrt{\frac{3M}{2M + m\sqrt{3}}}\) s, where \(T\) s is the period found in (b). [7 marks]

One end of a light elastic string, of natural length \(3l\) m, is attached to a fixed point \(O\). A particle of mass \(m\) kg is attached to the other end of the string. When the particle hangs freely in equilibrium, the string is extended by a length of \(l\) m. The particle is then pulled down through a further distance \(2l\) m and released from rest.

1. Prove that as long as the string is taut, the particle performs simple harmonic motion about its equilibrium position. [5 marks]
2. Show that the time between the release of the particle and the instant when the string becomes slack is \(\frac{2\pi}{3}\sqrt{\frac{l}{g}}\) s. [4 marks]
3. Find the greatest height reached by the particle above its point of release. [4 marks]
4. Show that the time \(T\) s taken to reach this greatest height from the moment of release is given by \(T = \left(\frac{2\pi}{3} + \sqrt{3}\right)\sqrt{\frac{l}{g}}\). [4 marks]