Like terminal velocity of an object falling in the air, is there the same said type terminal velocity for objects sinking in water? If so, how deep would an object have to be sinking in order to reach the speed? Does the weight of the object in water affect the speed as well?
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Terminal velocity applies to an object in any fluid. It depends on gravity, the fluid’s viscosity, the fluid’s density, the object’s weight, and its shape.
It’s the same idea as air, just that the specific numbers are different. Most objects a human deals with reach terminal water velocity in a under a second.
Yes, there is a terminal velocity in water. The velocity will be less in water compared to air.
The velocity will depend on the shape of the object and it’s density. So there is no fixed answer to “how deep”. Technically, it would be the density of the object rather than the weight that makes a difference but, for ELI5, weight matters.
>Does the weight of the object in water affect the speed as well?
The ratio of weight and friction forces is important. So yes weight matters
Changes in pressure as the object sinks would also alter this terminal velocity, as the object would become more dense from the water pressure. As water is rather incompressible, the objects density might rise even faster than that of water under pressure, making the object sink faster after a certain depth.
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>What is the max speed an object can sink in water?
There isn’t a reasonably easy answer to this. But, if you’re assuming this is all happening on Earth you can at least figure out a maximum theoretical speed. The acceleration due to gravity is -9.8 meters per second squared, and the most water you could fall through would probably be the deepest spot of the ocean, which is the Mariana Trench, which is approximately 11,000 meters deep. So, just from that, the maximum theoretical speed assuming you dropped the object from rest from the surface of the ocean, and the water didn’t impede it at all, would be about 464.5 meters per second (and it would take about 47 seconds to make that trip).
But, that’s not useful at all, because that kind of velocity isn’t practically attainable – the water is assuredly going to cause drag and reduce the actual acceleration of the object.
>Like terminal velocity of an object falling in the air, is there the same said type terminal velocity for objects sinking in water?
Yes, there is still going to be a terminal velocity. It’s a function of the same things as it is in air, too. Drag, mostly, which is much higher when the object is moving through a denser fluid. Buoyancy is also a thing; the object has to sink, and it has to do so with authority, which means it needs to be really dense.
>If so, how deep would an object have to be sinking in order to reach the speed?
This depends entirely on the object and its terminal velocity. The higher the terminal velocity, the more water column it’s going to need before it achieves that velocity. Remember, the *highest* amount of acceleration is -9.8 m/s^2. No matter what, the object can’t accelerate any faster than that, and it will need more and more distance to achieve higher and higher speeds (velocities).
>Does the weight of the object in water affect the speed as well?
Yes, and no. Weight is really just a practical result of mass in a gravitational field. Because the Earth has such great mass, the mass of any object we would be talking about here is comparatively negligible, and therefore the acceleration is going to be that -9.8 m/s^2. So, using weight as an indirect assessment of mass, the mass itself doesn’t really change much, but, practically speaking, the mass *in relation to other factors*, like the surface area of the face of the object oriented in the direction of sinking, that matters a great deal. Imagine a steel ball being dropped into the water, as compared to the same amount of steel formed into a spear. You should be able to intuitively see that that relationship between an object’s mass and its geometry does influence quite a bit about how it behaves underwater. Because there is a difference between these objects of equal mass, it *must* be true that a steel ball of at least the slightest bit of extra mass wouldn’t result in an object that would achieve a higher terminal velocity than a steel spear of slightly less mass. So, mass *directly* doesn’t contribute to higher terminal velocity, though it would considering equivalent geometries.
Also, water is sufficiently dense enough that its density in relation to the density of the object, ie buoyancy, plays a significant role in how objects sink. As an object’s mass is clearly related to its density, this facet has a profound effect as well.
Try to find the Myth busters episode on firing a gun into water. The bullets hit their non-lethal terminal velocity within a foot of entering the water.
Try this handy calculator:
https://www.omnicalculator.com/physics/terminal-velocity
For example: With a 1 kg sphere with a cross-sectional area of 10 cm^2, 0.47 coefficient of drag in seawater (~1020 kg/m^2), terminal velocity is 6.4 m/s. That’s 14.3 mph.
Optimal (teardrop Cd=0.04) shape? 49 mph.
The simple but unsatisfying answer is “the terminal velocity of that object in water”.
Terminal velocity in air is not a universal constant. Humans terminal velocity is something like 150 mph if they’re in the skydiving flat pose and up to about double that in the “pencil” pose. Feathers are more like 2 mph. The SR-71 in an unpowered dive probably very near the speed of sound.
Terminal velocity for all those objects in water would be probably an order of magnitude smaller, but still unequal to each other.
All i can add to this, is that as a freediver, we experience/cherish something we call “**free fall**” No propulsion, just letting gravity pull you towards the deep, once the pressure has compressed the air in the body enough, to give us negative buoyancy. We try to achieve “streamline”; being as long and slender as possible, arms close to the body, keeping a “straight line” with the body.
The speed is almost always **1m/s**.
That is also the speed we try to keep on the way up, because we don’t want to exert to much physical force, since it would lead to spending too much oxygen.
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