Some objects appear heavier underwater due to Archimedes' buoyancy force. This upward force exerted by the water compensates for some of the weight of the object, giving the impression that it is heavier.
When an object is submerged in water, it receives a kind of "boost" upward: this is Archimedes' principle. This phenomenon exists because water exerts an upward force on any submerged object, equal to the weight of the volume of water displaced by the object. This is exactly why we feel lighter in water. The more space an object occupies underwater (the larger its submerged volume), the stronger the upward buoyancy, making it appear lighter. Conversely, extremely dense objects like a stone or a piece of metal displace little water relative to their weight, so this buoyancy seems much less effective on them.
Each object has a different density, meaning that the material of an object can be more or less "tight" or compact. When an object has a density greater than that of water, it sinks, as it is overall heavier than the volume of water it displaces. On the other hand, an object with a lower relative density will float, because it is capable of displacing a quantity of water heavier than its own mass. Underwater, even if the object sinks, it still seems lighter than on land, as it partially benefits from Archimedes' buoyancy. Conversely, an object that floats will appear very light or nearly weightless underwater, because this buoyancy nearly fully compensates for its weight. Relative density thus explains why certain materials like polystyrene or wood seem to weigh almost nothing underwater, while others like steel still appear heavy, though a bit less than outside.
Hydrostatic pressure is simply the weight of the water above you exerting a force on your body when you dive. It increases proportionally with depth: the deeper you go, the stronger this pressure becomes, slightly compressing your body tissues. However, it acts equally from all directions at once, which balances the forces on your body and does not directly alter your actual weight. But this unusual sensation of compression, especially noticeable at significant depths, can influence your overall perception and give you the misleading impression of being heavier or compressed underwater.
When the water moves a lot around you, like with ocean currents or when there are waves, you can feel the weight of objects differently. If a current pushes upwards, you will feel as if the object is suddenly lighter. Conversely, a downward current can give you the sensation that an object is heavier. This makes sense, as moving water exerts additional forces on submerged bodies that either add to or oppose gravity. The same goes when you move an object yourself underwater: by moving it quickly, the water offers more resistance, making the object more tiring to handle, even though its weight hasn’t changed by a gram.
The perception of weight underwater largely depends on your brain. It anticipates how much an object should weigh based on your experience in the open air. This mental expectation can make an object surprisingly light or heavy underwater. Similarly, if you are tired or stressed during a dive, your brain may exaggerate the actual weight sensation of certain objects. Other factors also come into play, such as reduced visibility or the instinctive fear of depths, which enhance our physical sensitivity and alter the perception of weight.
The Dead Sea has such a high concentration of salt that people float very easily on its surface. This perfectly illustrates the crucial role of the relative density between the fluid and the submerged object.
Underwater, our visual perception is altered by light refraction. As a result, objects may appear closer or larger than they actually are, also influencing our perception of their weight.
Professional divers sometimes use weighted objects to perform specific tasks underwater. They must adjust their buoyancy with specialized vests to counteract Archimedes' buoyant force and work effectively.
The human body is made up of about 60% water, making it relatively close in density to fresh water, which explains why we tend to float easily after filling our lungs with air.
The temperature slightly alters the density of water: colder water is a bit denser, which slightly enhances the buoyant force of Archimedes. Although this difference is often imperceptible, a significant change in temperature can theoretically have a slight influence on the perceived weight of certain underwater objects.
Experienced divers know how to anticipate the differences in sensations caused by Archimedes' buoyancy and underwater movements, and are therefore less likely to be surprised or misled by these impressions. A beginner diver may perceive an object as heavier or more difficult to handle simply due to a lack of understanding of the specific parameters around them.
No, only objects with a density greater than that of water will appear lighter underwater due to the buoyant force of Archimedes. If the object is less dense than water, it will float, creating a sensation of weightlessness or even being pulled upward (positive buoyancy).
Directly, no, Archimedes' buoyancy does not depend on the depth itself, but rather on the displaced volume and the density of the liquid. However, at great depths, factors such as high hydrostatic pressure or increased resistance to any movement can indirectly influence our perception of weight.
This impression is mainly explained by the movements of the water and the turbulence generated when you handle an object underwater. The fluctuations in balance caused by these turbulences can create the sensation of an unstable weight, leading to the object oscillating in your hands.
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