The Mariana Trench is considered the deepest part of the oceans. It is about 11k (6.8 miles) deep and the pressure is over 1000 times the standard atmospheric pressure.
I’m on a ship directly over it and want to drop four objects overboard. The first object is a steel BB. The second object is a steel ball bearing about an inch in diameter. The third object is a steel sphere about the size of a bowling ball and the fourth object is a 1 ton steel spheer. All are made of the same grade of steel.
Will each of the objects reach bottom?
How long will it take for each object to complete it’s descent?
Will all of the objects decend at the same rate?
Will they descend at a constant rate? Will the minor increase in water density or the pressure slow the descent significantly?
What effect will ocean currents have on each object?
Is there anything else that would happen that is not apparent?
I’m not sure that’s true.
I would suspect that the BB-sized ball will take substantially longer than the 1-ton sphere to hit bottom. How long would a micron-sized sphere take? Surely not the same.
I don’t agree with this with respect to the speed. Hydrodynamic drag is going to act much more strongly on the BB, as its weight to frontal area ratio is the lowest.
Will each of the objects reach bottom?
Yes. Why wouldn’t they?
How long will it take for each object to complete it’s descent?
Depends on the size. They will all quickly approach their terminal velocity, so the approximate time is given below.
Will all of the objects decend at the same rate?
No. The density of the water will create a substantial drag force, limiting the velocity of the spheres. This limitation is a function of sphere size, because drag increases as a function of area, and weight increases as a function of volume (so larger spheres have a higher terminal velocity). My quick calculations (not guaranteed to be accurate; subject to peer review) have:
Object Terminal Velocity Time for Descent (36K ft)
BB 2.2 ft/sec 272 min
1" Ball 7.7 ft/sec 77 min
Bowling Ball 21 ft/sec 28 min
1 ton sphere 30 ft/sec 20 min
Will the minor increase in water density or the pressure slow the descent significantly?
No. Minor increases in density will have a minor effect; increases in pressure will have no effect (other than the effects due to water density change).
What effect will ocean currents have on each object?
Lateral motion.
Is there anything else that would happen that is not apparent?
I dunno. Sharks and giant squid will have an effect, I suppose.
oops, sorry. I was too fast in dismissing the whole idea that anything interesting would happen. this will give you a much better idea of how drag/terminal velocity works.
They wouldn’t if the pressure at some point causes the water to become more dense than steel. I think that’s what the OP is asking.
As to whether this is physically possible, I have no idea. On one hand, the density of steel is so high, I would doubt that water could ever reach that. On the other hand, even if it could, I would think that the same pressure acting on the water would act on the steel, and compress it, right?
If I can add my own question, would the steal balls be the same size on the ocean floor as they started on the boat? Is the pressure enough to compress a solid steal ball?
Well, in theory, giant transforming robots from space will be crushed at that pressure. The added cold water will help in ensuring they won’t be back to trouble us…until the sequel, that is.
Ah, I see. As a pretty good approximation, the bulk modulus of water (a measure of its resistance to compression) is 2.3 GPa, and the pressure at the bottom of the Marianas trench is 100MPa. Dividing the two, you would expect the density of seawater to increase by about 4% at that pressure – measurable, but not all that significant, and certainly nowhere near the ~780% increase needed to have water approach the density of steel.
The bulk modulus of steel is much higher than that of water: around 160GPa. Under only 100MPa of pressure, you would expect a decrease in diameter of less than 1/10 of 1%.
Well, to be honest, it depends strongly on the decade in which the measurement is being taken.
That seems plausible. Amend my previous statement to say, “Lateral motion, and change of terminal velocity depending on any vertical component of the current.” Or something like that.
