4 A uniform rod \(P Q\) has weight 18 N and length 20 cm . The end \(P\) rests against a rough vertical wall. A particle of weight 3 N is attached to the rod at a point 6 cm from \(P\). The rod is held in a horizontal position, perpendicular to the wall, by a light inextensible string attached to the rod at \(Q\) and to a point \(R\) on the wall vertically above \(P\), as shown in the diagram. The string is inclined at an angle \(\theta\) to the horizontal, where \(\sin \theta = \frac { 3 } { 5 }\). The system is in limiting equilibrium.

1. Find the tension in the string.
2. Find the magnitude of the force exerted by the wall on the rod.
3. Find the coefficient of friction between the wall and the rod.