Ever wondered why a heavy metal ship floats, but a tiny coin sinks? Why do some fruits float, while others don’t?
To get to the bottom of these mysteries, you need to know a little more about the science of density and buoyancy. This means exploring the fascinating behaviour of everyday objects in water.
In this post:
Key Takeaways
Floating is mainly determined by scientific principles of density, mass, and volume
Objects float when they displace enough fluid to counteract their weight
Everyday examples – from ships to balloons, fruit to fizzy drinks cans – show these principles in action
Water type (saltwater vs. freshwater) affects buoyancy too
Understanding the Science of Floating and Sinking
The reason that some objects float and others sink is all down to scientific principles relating to density. You’d be forgiven for thinking that weight is the deciding factor, where heavy things sink and lighter things sink.
However, the reality is a little more nuanced. It’s all to do with density, which is the relationship between the mass of an object and the space it takes up.
The Role of Mass, Volume, and Density
Density refers to how tightly packed an object’s matter is. It can be calculated using a simple formula – density = mass ÷ volume.
To understand this and how it affects floating, it’s helpful to picture some real-world examples.
Take a coin, for example. It’s small, heavy, and has high density – where a lot of mass is packed into a small space. Due to this, the coin sinks.
A large ship, on the other hand, is mostly hollow. This means it has a lower density and crucially, this density is less than water. This is what allows it to float.
How Buoyancy and Displacement Work
Another important principle to get to grips with is buoyancy, or more specifically – the Archimedes Principle.
This principle states that an object submerged in fluid will experience an upward buoyant force which is equal to the weight of fluid it displaces.
An easy way to picture this is imagining yourself having a bath. When you sit in the water, your body displaces the water. This also gives you a feeling of lightness or buoyancy.
Common Misconceptions About Floating Objects
Now it’s time for some myth-busting. Let’s run through a few of the common misconceptions about floating objects:
- “Heavy things sink, while light things float” – this may seem true, if you compare the buoyancy of a bowling ball (heavy) to a beach ball (light) However, it isn’t true in all cases, as a huge, heavy iceberg floats while a small rock sinks
- “Air-filled objects float, because they weigh nothing” – in fact, air does have mass and density, but it’s simply much lighter than water
- “ Salt magically makes things float” – this one is actually true, although it’s science rather than magic at play. Salt increases water density, which in turn increases buoyant force
Everyday Examples of Density in Action
Why Heavy Ships Float but Small Coins Sink
The classic example of density in action – and one we’ve already looked at a little so far – is a heavy ship vs. a small coin.
The ship may weigh thousands or even tens of thousands of tonnes, but the design ensures that it will still float. This is because of the hollow hull, which is filled mostly with air. This lowers the average density of the ship. It also displaces enough water to counteract its weight.
A coin, meanwhile, is tiny and solid. It displaces very little water, so its density exceeds that of water and it sinks immediately.
Floating in Saltwater Versus Freshwater
Interestingly, the type of water also affects whether or not objects float. A prime example of this is saltwater in the ocean, which is denser than freshwater because dissolved salts add mass without significantly increasing volume.
This is why, for example, you may float in the Dead Sea, due to the salt content of the water.
Density effects are often studied using deionised water, as it has no dissolved minerals and can therefore provide a consistent baseline for scientific experiments.
Balloons, Air, and the Concept of Floating in Gases
When we talk about fluids in relation to floating and displacement, we can also include gases.
Helium balloons rise because helium is less dense than air, while hot air balloons float because heating the air inside makes it less dense than the surrounding cooler air.
Even smoke rising from a chimney is a demonstration of this principle, where less dense hot gases move upwards through the denser surrounding air.
Surprising Objects That Challenge Expectations
There are some objects which do almost the opposite of what you expect when it comes to floating or sinking. Here are some example experiments where the results may surprise you:
- If you peel an orange, it’ll sink, whereas an unpeeled orange will float. This is because the peel traps air and reduces average density
- Put two cans of fizzy drink in water – one diet, and one regular. You’ll notice that the regular one sinks, but the diet soda can will float due to the artificial sweeteners (compared to ordinary sugar) changing its density
- Test out putting pencils, apples, and candles in water – you should find that they float, but it depends on their material and shape
Real-World Applications of Density and Buoyancy
Let’s take a look at a few of the real-world industries and applications where principles of density and buoyancy play a key role:
- Engineering and shipbuilding – hull design optimises displacement and stability of ships, while in submarines ballast tanks are used for sinking and rising
- Environmental science – oil spills float due to lower density, which can help with clean-up strategies
Aviation – hot-air balloons, fuel tanks, and airships all rely on density differences - Food and drink – even the layers in your favourite cocktail are created via the interplay between liquids of different densities
Conclusion
Floating isn’t about weight or size, but density and buoyancy. From icebergs to balloons, the same principles explain why some objects float and others sink. Understanding these helps us to make sense of everyday phenomena and the science behind the world around us.
