Have you ever wondered why some people float well while others don't, or why massive, very heavy ships float at all? Then this should be a good read for you!
Buoyancy is determined by the following factors:
The density of a fluid
The effect of gravity on an object
The volume of fluid displaced by the object
Density is a measure of how "packed" something is, specifically a thing's mass divided by its volume.
Photo Credit Yupi666, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons
A diagram to picture how buoyancy works.
The "rule of float" is this: for an object to float in a fluid, it must displace an amount of fluid that matches or exceeds its own density.
Ice and oil float on liquid water because they are less dense than liquid water.
You may have heard or experienced that people float better in the ocean than a typical swimming pool. That's because saltwater is more dense than freshwater, and a person's body (or any object, for that matter) does not have to displace as much water to achieve positive buoyancy. The effect of this is that when a person jumps into the salty ocean instead of a typical, freshwater swimming pool, they float higher on the water's surface.
Photo Credit Shai Bitton, CC BY-SA 3.0, via Wikimedia Commons
In this photo, the diver on the left is teaching the diver on the right how to achieve neutral buoyancy.
For every scuba diver, a combination of personal buoyancy and specialized equipment is what allows them to dive at all. Scuba divers wear BCDs (or BCs), also known as buoyancy control devices, which are backpacks that can be inflated with air. When a diver inflates their BCD, the device expands with pressurized air and increases their buoyancy. The effect of adding air to a BCD is dramatic and can make a diver very positively buoyant very quickly, since the volume of the BCD was greatly increased with one of the most lightweight materials in existence, air--in other words, the density of the diver and their equipment was largely decreased while simultaneously displacing more water. Because the extra air in their BCD is pressurized, the diver can just as easily release that air too, deflating their BCD and decreasing their buoyancy. Even breathing changes buoyancy appreciably; I've experienced this myself while diving! We don't experience this because people normally are positively buoyant enough to render the influence of breathing unnoticeable. Divers actually add weights onto their equipment before diving so they can be slightly negatively buoyant when their BC is deflated, allowing them to sink below the water's surface at the beginning of each dive. Scuba divers are unique in that whenever they dive, they can cancel the effects of gravity with buoyancy--whenever they become neutrally buoyant in this way, they experience weightlessness!
Well-fashioned boats float on water, no matter how many tons they weigh, because once they sink into the water to a certain level, they'll have displaced enough water to satisfy the above-given "rule of float".
Photo credit Jacques Girard, Copyright-free use, via Wikimedia Commons
This ship is Batillus, one of the "Batillus Class" supertankers (oil vessels) and among the top five longest and heaviest ships ever made at 414.22 meters (1,359 feet) long and 275,268 tons. It floats fine!
Buoyancy doesn't just apply to things in water: the term "fluid" that has been used throughout applies to any other liquid or gas that exists.
Photo Credit Klim Levene, CC BY 2.0 https://creativecommons.org/licenses/by/2.0 via Wikimedia Commons
Even a hot air balloon rising in the atmosphere can be called "positively buoyant" because it really is floating on air!
Photo Credit NASA Dennis Keim, Public domain, via Wikimedia Commons
Astronauts practice moving around and working on equipment while neutrally buoyant in large pools to get used to the weightlessness of outer space before doing the real thing on missions! Notice the scuba divers in the background helping out and the bubbles that give away the presence of the very clear surrounding water.
Not being able to float doesn't mean a person is overweight or unfit to swim. I've been on the skinny side all my life and scuba dive, and I don't float properly like Batillus does unless I have scuba gear on to add some buoyancy! Everyone's body structures and densities are different from any other person's, so one person's natural buoyancy often varies from another's.
Sources:
The science of buoyancy
Article on the world's largest ships
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