There are multiple explanations I can give for this, but let me start with the simplest one.
James Cameron wanted Jack to die - in fact, it was essential for the success of the movie that Jack dies in the end. You see, happy endings don't really sell well; so, Jack had to die for James to win all those Oscars he received for the movie.
Secondly, I think James Cameron is a perfectionist of perfectionists, and he would have thought of this problem too; he isn't stupid after all, and the most relevant explanation I am sure he could have come up is given by physics: and it's called Buoyancy. It's not about how much room there is on the plank, it's about how much weight it can take.
Buoyancy is an upward force exerted by a fluid, that opposes the weight of an immersed object.
Now, Buoyant Force = Volume of fluid Displaced x Gravitational Acceleration x Density of the Fluid
In layman terms, if buoyant force exerted by the fluid on you is equal to the force you exert on the fluid - which is basically your weight (mass * gravitational acceleration) - then, you float; else you sink.
Let's go over it once more :
Buoyant force = weight => float, good.
Buoyant force < weight => sink, bad.
As depicted in the picture, the length of the plank exceeds Kate Winslet's by, say, half a meter, Wikipedia tells me that Kate Winslet stands tall at 169 cm (); add to that another 50 cm and round it off to about 220 cm or 2.2 m. Breadth of the plank can be assumed to be a meter (give or take a few cm); a standard door as I can see from the measurements of my door is about 5 cm thick. Since it is the Titanic we are talking about here, I'll add a couple of cms to that thickness.
So as we have it:
Volume of water displaced by the plank(assuming it is fully submerged) = the volume of the plank = 2.2 x 1 x .07 = 0.154 m³.
Density of ocean water(salt water) = 1020 Kg/m³ ()
Gravitational acceleration = 9.8 m/s2
=> Buoyant force = 2.2 x 1 x .07 x 9.8 x 1020 = 1539.384 ~ 1540 N
now they will float if the total force exerted by the three bodies(Jack, Rose, and the door/plank) is less than this buoyant force.
Force exerted by the plank(weight) = Density of the plank(wood) x Gravitational acceleration x Volume of the plank
*density of wood = 650 Kg/m³ ()
=> Force exerted by the plank(weight) = 650 x 9.8 x 0.154 = 980.89 ~ 980 N
=> 1540 - 980 = 560 N
Now, 560/9.8 = 57.142 Kg
This means that in addition to its own weight, the plank could only support another 57.142 Kg and I am fairly certain that Rose must have at least weighed 45-50 Kg - she was a healthy girl after all, if you know what I mean.
So there you have it, it's not about the room on the plank, its about how much weight it could support. Think physics, people.
More questions on Movie Story and Plot Themes:
- What are the plot holes and realistic problems with the invasion in Olympus Has Fallen?
- Why did Hendrick ask where Liam Neeson was going that last night at the bar?
- Which movies had outstanding story lines but poor performances by the lead actors/actresses?