Have you ever wondered about the secret of floating and sinking? Why does a huge ship float on the sea surface whereas a tiny needle sinks in the blink of an eye? It might seem some sort of magic, but in reality, there are precise scientific laws that govern such phenomenon. In this article, we will embark together on a journey into the world of physics, to learn about Archimedes' Principle and discover how density and shape play a fundamental role in determining the fate of any object placed in water.
What is the difference between buoyancy and submergence?
Buoyancy means the tendency of an object to float or rise in a fluid when submerged; this fluid can be either a liquid (such as water) or a gas (such as air). This happens when the force of the fluid pushes the object upwards; this force is called the "buoyant force" (also known as "upthrust" or "Archimedes' force"). An object will float if this force is greater than or equal to its weight, and will sink if it is less than its weight.
The Eureka of Archimedes
More than two thousand years ago, the Greek scientist Archimedes discovered this principle upon stepping into the bathtub; he noted that his body felt lighter. He realized that the volume of water displaced is equal to the volume of the submerged object. This displaced water tends to return to its original position; thus, it exerts an upward force on the object, known as the "upthrust" or "buoyant force".
The greater the amount of water displaced, the greater the upward (buoyant) force. An object will float if the upward buoyant force is greater than or equal to the gravitational force (or object's weight) acting on it.
Archimedes' Principle: Buoyant Force = Weight of the Fluid Displaced by the Object.
Role of Density and Shape in Buoyancy
- Buoyancy is closely tied to density, which is defined as: the ratio of an object's mass to the volume it occupies.
- The density of water is about 1 gram per cubic centimeter (1 g/cm3); the ratio of the density of an object to the density of water, is known as "specific gravity".
- If the object's density is less than that of water, such as wood or plastic, it will float.
- If the object's density is greater than that of water, such as iron or stones, it will sink.
- Most floating objects are bodies with a relatively large volume and low density.
Why ships float and needles sink?
Ships are made of heavy iron and have a hollow, bowl-like design that displaces a large amount of water when placed in the sea. This generates a buoyant force greater than the ship's weight; thus, keeping it afloat. It is not just a matter of the metal's volume, but rather the shape and the distribution of the weight of an object.
Unlike the ship, a needle is tiny, solid, and has a high density; it displaces a small amount of water when dropped into it. This action fails to generate enough buoyant force to counteract the needle's weight, and it instantly sinks.
How do fish maintain buoyancy?
Fish maintain their buoyancy thanks to an organ known as the "swim bladder"; an organ shaped like an air-inflated balloon. It expands and contracts to adjust the fish's position as it rises or descends in the water. When the bladder expands, the fish's volume increases while keeping its mass unchanged. This decreases its specific gravity, which causes the fish to rise. A decrease in the bladder's volume leads to an increase in its specific gravity, which causes the fish to descend.
Buoyancy in Our Daily Lives
Buoyancy is not limited to just ships; we can see it in our daily lives in examples, such as:
- Swimming: when we swim, our bodies displace water, which generates the buoyant force that helps us stay afloat.
- Hot Air Balloons: buoyancy applies to gases just as it does in water; a helium-filled balloon floats in the air simply because its density is less than that of the surrounding air.
- Submarines: these vessels can dive and ascend by controlling their weight; they fill their tanks with water to dive, and empty to float again.
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Buoyancy is a fascinating scientific secret, which explains how some objects afloat while others sink. Without Archimedes' principle, people would never have uncovered this secret, nor would have built giant ships, enjoyed swimming, or even sent submarines to the deep sea.
References
britannica.com
courses.lumenlearning.com
sciencelearn.org.nz
Cover Photo by Freepik.