11 Two balls \(P\) and \(Q\) have masses 0.6 kg and 0.4 kg respectively. The balls are attached to the ends of a string. The string passes over a pulley which is fixed at the edge of a rough horizontal surface. Ball \(P\) is held at rest on the surface 2 m from the pulley. Ball \(Q\) hangs vertically below the pulley. Ball \(Q\) is attached to a third ball \(R\) of mass \(m \mathrm {~kg}\) by another string and \(R\) hangs vertically below \(Q\) (see diagram).
\includegraphics[max width=\textwidth, alt={}, center]{8c0b68bd-2257-4994-b444-def0b3f64334-7_419_945_493_246}
The system is released from rest with the strings taut. Ball \(P\) moves towards the pulley with acceleration \(3.5 \mathrm {~ms} ^ { - 2 }\) and a constant frictional force of magnitude 4.5 N opposes the motion of \(P\).
The balls are modelled as particles, the pulley is modelled as being small and smooth, and the strings are modelled as being light and inextensible.
- By considering the motion of \(P\), find the tension in the string connecting \(P\) and \(Q\).
- Hence determine the value of \(m\). Give your answer correct to \(\mathbf { 3 }\) significant figures.
When the balls have been in motion for 0.4 seconds the string connecting \(Q\) and \(R\) breaks.
- Show that, according to the model, \(P\) does not reach the pulley.
It is given that in fact ball \(P\) does reach the pulley.
- Identify one factor in the modelling that could account for this difference.