How Do Things Float On Water?

Have you ever wondered how boats stay afloat or why ice floats on a glass of water despite being made out of the same substance? Why does an unpeeled orange float while peeled oranges sink?

The truth is, floating isn’t necessarily a one-science matter. There are several principles and phenomena at play as matter float. Many scientific factors contribute to the capability of certain things to float, affecting not only those that float but also the very water it floats or sinks in.

Before moving forward, here are some misconceptions that have to be addressed before you move forward:

  • Many students think that heavy objects automatically sink and light objects naturally float regardless of the size, shape, or physical properties of the material that constitute them.
  • The concept of floating is exclusively limited to objects wholly positioned above the surface of the liquid
  • Air is absolutely needed in floating, thus, all objects that float surely contains some form of trapped air and that is the only reason they float
  • Materials float more effectively on vast amounts of water, meaning that the amount of liquid on which an object floats bears a significant impact on whether things float or not (i.e. objects will float better in a larger lake or pool or deeper body of liquid).

It’s very important to note that these notions are mere misconceptions and are false. There’s more to objects at float than meets the eye. Now that the misconceptions are clear, it’s time to dive into the nitty-gritty of the science of floating.


While there is definitely a plethora of scientific principles empowering matter in their ability to stay afloat on water, one concept that would certainly be at the forefront of any scientist or professional’s hypothesis or inference would be density.

Density, as eloquently explained by Brittanica, is the “physical property is defined as the ratio of mass to volume of a substance.” In other words, it is the element of matter that deals with how tightly packed together the molecules of an object are.

Quick experiment: on a basin or pail of water, drop a big block of wood onto the water and a small pebble and record your findings. Surprisingly, science has shown that 10 times out of 10, the wooden block floats, while the pebble of rock sinks promptly.

Why is this the case? To uncover why a block of wood with a mass much larger than the pebble floats while the small splinter of rock sinks, you must look deep inside each object, understanding its composition at a molecular level.

Everything around you or anyone is made out of molecules. Molecules are tiny lego blocks that make up everything and everyone in the world. They are so small these building blocks that constitute the world can only be perceived through a special microscope. For some objects, like the pebble you used in the quick experiment, molecules are packed tightly together. But in others, like the wooden block that floated, the molecules are more spread out. This is density – the physical property that explains and measures how closely molecules are packed together in a certain space. Density is one of the biggest factors that allow objects to float.

The Science of Density

Dense objects such as coins, rocks, marbles, and steel bars are much denser than water. This will make them sink. On the other hand, objects like apples, wood, and sponges are less dense than water, so they float. It is also important to note that density is not limited to liquid and solids. Air also plays a big part in enabling matter to float. Hollow things like empty bottles, balls, and balloons are also going to float due to the air that resides inside them. This is because the air itself inside these hollow objects is much less dense than water. This is one of the primary reasons why ships are able to stay afloat. Despite being extremely heavy, ships still don’t sink because of the air in their hollow chambers.

Scientifically speaking, the density of any substance can be determined by dividing the mass of the object by its volume. Generally, mass is often measured in grams (g) while volume is measured in cubic centimeters (cm3) or milliliters (mL).

Density explains how some objects float while others sink in the water despite a deceptive disparity in mass and overall size. The logic is as follows: whichever object has less density is to float in a liquid that is denser. For one, oil, a liquid, will float on water because it still has less density than water. A large steel frigate, despite its gargantuan size and mass, still floats on water. Even though the steel constitution of the ship is denser than water, most of the ship’s space is still filled with air, which is less dense than water. Therefore, the overall density of the ship, then, is much less than that of the water, allowing the ship to float at sea.

Shape and Buoyancy

But density is far from the only scientific element that enables massive machinery to float on water. The shape of objects also plays a share in the sea-faring capability of materials and objects. A quick experiment you can try is the tinfoil ship project. The premise is easy: you make two boats that differ in shape. One boat will be a flat vessel while one will be shaped like a ship. Add some marbles into the boats one by one and see which one sinks first. If the science is correct, one of the vessels will sink before the other. The surviving vessel will be able to carry more marbles than the one that sank. In hindsight, you used tinfoil, the same substance, for both vessels. You used the same load, marbles, on both vessels yet one of them sank first. The differing variable in this experiment is the shape.

In this same experiment, one more scientific principle comes into play: buoyancy. As more of the surface of an object is touching the water, it is more buoyant. This makes it easier for objects to float. The logic here can be attributed to one of Sir Isaac Newton’s Laws of Motion. As an object floats, it pushes the water out of the way. In this same instance, as the surface of the boat covers a greater area, it pushes a greater amount of force onto the water, causing the water to be pushed out the way. This is called displacement.

When the object pushes onto water, however, water pushes back, accurately exhibiting the second law of motion that asserts that for every action, there is an equal and opposite reaction. These elements also help in making objects float on water.

These are just a few of the scientific principles that impact the capability of objects to float in water. There are many other forces out there that influence the floating capabilities of matter but these are the primary elements that explain and make it possible. 

Elena Jones

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