Uh, what? Buoyant objects do not suddenly become non-buoyant objects simply because they have weight exerted on them. Unless the buoyant object is somehow able to fill up with water, it will never sink, not until the wood is broken down and decomposed. Now, if Kate had moved over and made room for Jack, and they had, for example, sat side by side on the wood, the door would probably sink a few inches below the surface due to the depression of weight on its surface, but it would not sink to the bottom of the ocean. You can test this theory with a piece of wood in your bathtub, if you like.

Now, the question is not really whether there is room for Jack, but whether Jack is a good person. Since Jack is a good person, and probably aware that joining his ladyfriend on the plank would, in fact, make it sink a few the inches below the surface, he is also aware that A) that wouldn’t help him, anyway, because he’s freezing to death and the only solution not to freeze to death is to remove himself from the water altogether and B) ladyfriend would probably freeze to death, as well, because they would now both be partially submerged in water. Since Jack is a good person, he’s not going to attempt making room for two.

And this is all assuming your calculations are correct.

(Source: dreamingofsomewhere)