4. A lifeboat slides down a straight ramp inclined at an angle of \(15 ^ { \circ }\) to the horizontal. The lifeboat has mass 800 kg and the length of the ramp is 50 m . The lifeboat is released from rest at the top of the ramp and is moving with a speed of \(12.6 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) when it reaches the end of the ramp. By modelling the lifeboat as a particle and the ramp as a rough inclined plane, find the coefficient of friction between the lifeboat and the ramp.
(9)