Objects appear lighter in water than in air due to Archimedes' buoyant force. This buoyancy force acts opposite to the object's weight, thereby reducing the effect of gravity and giving the impression that the object is lighter.
When you immerse an object in a liquid, it experiences an upward force called buoyancy. Essentially, this means that a submerged object appears lighter because the liquid "pushes" it upwards. This buoyancy is equal to the weight of the displaced volume of liquid. The more water an object displaces, the greater the upward buoyancy. That’s why, for example, a large bottle full of air will struggle to stay at the bottom, even if you push it down with your hand. It is pushed upwards more strongly by the water than it actually weighs. Conversely, a small, very dense stone will sink without issue because the buoyancy is weaker than its actual weight.
Water is way denser than air, about 800 times more. Imagine pushing your hand into a swimming pool; it resists much more than just waving it in the open air. Underwater, every liter displaced weighs much heavier than in the open air, creating an upward force. It's precisely this difference in density that makes water push us upward and thus reduces the sensation of weight: you float better in water because it is simply denser, more compact than the air around you.
The real weight of an object simply corresponds to the force with which the Earth attracts it: the more massive an object is, the greater this force is. But as soon as you place an object in a fluid (like water), you experience an apparent weight, which means the sensation that the object is lighter than it actually is. Why? Because the force of gravity remains the same, but another force acting in the opposite direction comes into play: the famous Archimedes' buoyancy. This buoyancy acts upwards, countering gravity, and thus reduces the sensation of weight felt. The result? It feels like the object has become lighter once submerged. The higher the density of the liquid you immerse yourself in, the stronger the buoyancy is. That's why you float much better in saltwater (like the sea) than in freshwater!
Buoyancy directly depends on the submerged volume of the object and its density. The larger the volume of an object, the more water it displaces, resulting in a significant upward force. If its density is low, like that of an inflatable balloon, this force can exceed its actual weight, allowing it to easily float on the surface. Conversely, a very dense object (high density), even if small, will struggle to stay on the surface because the upward force of the water will not sufficiently counterbalance its weight. That’s why a small metal ball sinks directly, while a large plastic buoy floats calmly.
When you take a heavy object, like a large stone, and submerge it in water, you immediately feel that its apparent weight is reduced, as if someone is discreetly helping to carry it. Another classic example: in a swimming pool, we float easily, while in the air, obviously, we fall right away. This also explains why moving a large piece of furniture or heavy objects underwater, for those who dive, is much easier than on the street in open air. Instinctively, everyone notices this feeling of lightness when swimming or simply holding a submerged ball: the more you push the ball underwater, the stronger you feel this upward force, known as Archimedes' buoyancy.
Fish possess a swim bladder filled with gas, allowing them to easily regulate their buoyancy, remain at depth without effort, and thereby conserve their energy.
Astronauts frequently train in deep pools to simulate the weightlessness they experience in space, an experience made possible precisely by Archimedes' buoyancy!
Archimedes' principle also acts in the air, but it is much weaker due to the low density of air compared to that of water. This is why we rarely feel this buoyancy in our daily lives!
Submarines can dive and resurface by simply adjusting their density: they take in water into tanks to sink and expel this water to rise.
The apparent weight does indeed change due to Archimedes' buoyancy, a real force exerted by the water upward. Thus, an object submerged actually experiences less downward force, reducing the apparent weight felt when it is immersed in water.
It depends on the density of the object compared to the density of water: a denser object sinks, while a less dense object floats. Density is calculated by dividing its mass by its volume. If it is greater than that of water (about 1 g/cm³), the object will sink.
Water exerts a significant Archimedes' buoyant force on the submerged person, greatly reducing their apparent weight. This allows you to carry them much more easily in water than in air.
Yes, slightly. When heating water, its density decreases because it expands. Thus, warm water will be less dense, slightly reducing buoyancy, although this effect is generally minimal for moderate temperature variations.
You float more easily in seawater because it has a higher density due to the presence of dissolved salt. This higher density results in a greater buoyant force, supporting your body more.
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