Forces?

I am unsure how to answer this question (Q18 part d(ii)).
What is the concept behind it?
https://c181ae78b6fc6200ce96232927a65efd1e29cb83.googledrive.com/host/0B1ZiqBksUHNYc0tWVkExdEc4Nlk/January%202009%20QP%20-%20Unit%201%20Edexcel%20Physics.pdf

Which bit are you exactly referring to, the labelling, the equations or the weight?

The question just after it asks you to explain what compressive strain is.
It asks on what action should be taken when the pressure increases as the submarine goes further in depth.

So 'compressive strain', is just strain. It's defined as change in length divided by length. In the case of compressive strain, the pressure is trying to decrease the size of the submarine. Strain is given by the formula $\epsilon = \dfrac{\Delta l}{l}$

When the volume of the submarine decreases, its density increases, which will cause it to sink from its constant depth. What could you do to decrease the density?

You would have to reduce weight of submarine? However, If the mass is not reduced would it have a direct effect on balanced forces (Weight of Submarine and Upthrust)?

What exactly do you mean? If the mass is not reduced, the density will continue to increase. Do you understand the idea of sinking due to density in water?

No I haven't come across it :/

it has a greater density than water?

Indeed. For the submarine to be at a constant depth, the density of the sub must be the same as the density of the water. If the density of the sub is increasing, you must in simple terms, reduce the mass of the sub, to lower its density back down. I guess you could just remove water from it to achieve this.

You could think about this with forces too, and the smaller sub will have a smaller upwards thrust from the water due to the compression. Hence, the weight downwards will be greater than this thrust. To balance these again, you need to reduce the mass.