1. A large rock with a mass of 230 kg is tied to a buoy by a light inextensible rope and is then thrown into the water.  The rock is heavy enough so that as it sinks vertically downwards it pulls the buoy through the water behind it. 

As the rock sinks through the water, it experiences a resistance to its motion that may be modelled as a constant upwards force of 150 N. 

The buoy has a mass of 10 kg, and the combination of its buoyancy and the water resistance to its movement may be modelled by a constant upwards force of $P$ N.

Given that the tension in the rope as the rock sinks is 655 N, determine the value of $P$.