# Can a submerged object in a free falling fluid sink faster than falling?

I've been wondering about how sunken objects would behave if you could instantaneously flip their container with water. Like if you had a bucket filled with normal tap water and you dropped a ball in it. Then SOMEHOW you instantly flip the bucket over such that for just a moment, the water is suspended in midair and the ball is still touching the base of the bucket.

What I want to know is, first of all what would happen? I know that if you drop a bowling ball and a bouncy ball from the same height they'd just about reach the ground at the same time, but what about this scenario? If you "drop" water and an object inside the water at the same height, do they reach the ground simultaneously? Or does the object "float" down the falling water?

Second of all, under what conditions could the object fall first? That's what I'm looking for- what kind of object could fall out of the water and reach the ground before the water does? What difference would there be between the object being a sunken wooden ship and a rock?

Third of all, say we take this experiment to the ocean, where the water pressure gets higher as you go down. If there's an object at the bottom of the ocean and we flip the ocean, how does the pressure affect things?

• I think this is interesting, but your third question seems to be redundant. All fluids increase pressure with depth, and that's actually how buoyancy works in the first place. Although the pressure at the bottom of the ocean is way different than the pressure near the surface, the pressure difference due to height only changes a bit due to density.
– JMac
Jan 24, 2020 at 18:15
• Does the base of the bucket also instantly disappear the moment the bucket is flipped? Jan 27, 2020 at 14:15
• Othe, the simple way to understand this is just that each "atom" acts on it's own normally as it would fe nothing else was there. It would all just "fall down". In your third image everything would just look like that forever, until it all hit the ground. Your experiment is literally identical to the hammer-feather thing. (Of course, air resistance etc. affects things, but that's normal, and not what you're asking about.) Jan 27, 2020 at 17:08

I want to supplement Emilio's intuitive answer discussing what would happen with some thoughts as to why what you propose in your second part cannot happen.

What kind of object could fall out of the water and reach the ground before the water does?

Let's assume the water is a single entity. In order for the object to accelerate faster than the water, the object needs to have a larger downward acceleration than the water does. This would need to come from a downward net force that is larger than the weight of the object, as in free fall both objects will have the same downward acceleration. Where would this force come from? There is no buoyant force in a free-falling fluid, but even if there were the buoyant force would act upwards on the object, not downwards. Therefore, the best your could even hope for is that the object and the water move together, and this is indeed what happens.

Perhaps the confusion comes from why certain objects normally sink. They aren't pulled down by the water, they are pulled down by gravity. In your everyday scenarios it is just that the fluid impedes this "falling". You could even think of us as all sinking in the Earth's atmosphere. Therefore, in your scenario, it is not the case that because the object is in water suddenly means it wants to move down through the water. The water itself is not the mechanism for why objects sink.

The Equivalence Principle tells you that physics in free-fall is identical to physics in an inertial frame in the absence of gravity, in which case there is no buoyancy and any changes to the boundary of the liquid come from surface tension and related effects. The same is true for your falling mass of water.

• Hi, I don't understand much of what you said, could you dumb it down please? Jan 24, 2020 at 18:23
• @OtheJared - with no gravity there's no buoyancy. If there's no buoyancy, then nothing can "sink"; there's no direction for things to sink to. - E.g., a bottle of water in outer space with a tennis ball in it: it will bounce around in there every which way. But if you bring the bottle back into a gravity field, buoyancy {due to the large volume (the size) of the ball, relative to it's mass (its weight) compared to that of water} ... then it floats. - A lead ball would sink on earth, but it'd also bounce around up there. Jan 25, 2020 at 3:01
• If the water is falling through air, it won't be in free fall - it will reach terminal velocity. Then the ball would fall through the water, right? I'm not sure if it would fall entirely out of the water, though - could its own terminal velocity be lower than that of the blob of water? Jan 27, 2020 at 10:09
• Terminal velocity implies that the upward force (due to air resistance) matches that of gravity similar to how the upward (normal) force of the ground matches that of gravity -- either way, net acceleration is zero. Thus sinking and floating would behave as they normally would. What happens when the object falls out of the blob is another matter entirely. Jan 27, 2020 at 21:33

under what conditions could the object fall first?

Air resistance.

If there is enough air resistance to slow down the water, then the denser than water object will fall faster than the water.

Except OP said the liquid was free falling, so air resistance doesn't apply here

flip the ocean, how does the pressure affect things

This is quite different.

For one thing, it's not free-falling.

The high pressure water that is now at the top will expand - some of the water at the top will initialy be pushed up, possibly with enough force to lift the heavier-than-water object. It will then fall. Water not at the top will fall from the instant the sea is 'flipped', though perhaps sink is a better word because this isn't free-fall, it's compression (at the bottom) and it's decompression (at the top).

There would be more than a bit of bouncing and turbulence, but before too long, the original pressure distribution will be restored.

Meanwhile, possibly after having initially been thrown upwards, the heavier than water object will again sink to the bottom. Depending on friction, the object might drag down some water with it, but most of the water at the top will stay more or less the top, and most of the water at the bottom will stay somewhere near the bottom - until temperature convection and other causes of turbulence come into play. But I think that's going beyond the scope of the original question.

once you tip the bucket over the water,ball system is in free-fall.

in free-fall gravitational forces have no effect on the arrangement (except for tidal effects which a very weak at this scale) so everything falls at the same speed. you have a blob of water with a ball near the top.

what could cause the ball to move faster.

• Springiness, if the ball was slightly squashed at the bottom of the bucket it springs off the bottom of the bucket moving slightly faster than the water

• Magnets, if the ball was made of some magnetic substance (like steel) it could be attracted to a nearby magnet. water is also slightly repelled by magnets.

• Currents. Atmospheric drag will push the outer layers of water backwards creating a current in the water that moved it backwards behind the ball.

Third of all, say we take this experiment to the ocean, [...] and we flip the ocean, how does the pressure affect things?

The pressure goes away as soon as you flip. pressurised things at the bottom of the ocean expand suddenly. otherwise same.