Why Do Things Float In Water? Fun Physics Facts For Kids

You must have noticed that certain objects float in water, whereas some objects do not.

Usually, objects which are less dense than water will float when immersed in it. To understand why things float and what types of objects will float in water, first we must understand the concept of density.

Read to understand how density decides which object floats or sinks. If you enjoy this, check out why do lights flicker and why do clothes shrink. 

What types of things float in water?

What is density? Everything around us, the moon, the stars, buildings - even you and I - are made out of tiny particles called molecules. However, the difference in various objects lies in how tightly these molecules are packed together.

Liquid objects can 'flow' as the molecules in liquids are much less tightly packed together than those of solids. Solids can maintain their shape as the molecules are very tightly packed, meaning that solids have a very high density. Humans can also float.

Though liquids such as water generally have less density than solids, this depends on the volume of the water as well as the object of the solid.

For example, a person would sink to the bottom in a small tub, however, they would float in a pool or in the vast sea. Another concept to keep in mind here is buoyancy: the upward push which helps to keep objects afloat on the water.

In a way, buoyancy is like the opposite of gravity, gravity pulls objects down, while buoyancy pushes them upwards.

The buoyant force exerted depends on weight of the liquid displaced by putting the object into the water, as well as the volume of liquid you are immersing it in. If the weight of an object is more than the buoyant force exerted upon it, it sinks.

If the buoyant force push is bigger than the weight of an object, it will rise to the bottom of the liquid and stay there, bobbing on the surface.

In simpler words, an object sinks into the water until it displaces enough water equal to its weight, which is matched equally from below by buoyant forces.

This is how ships can float in the ocean, as the water displaced by the ship does not make a dent compared to the large volume of the sea, hence it cannot sink unless it is overloaded or damaged in any way.

The upward push will keep increasing until it matches the object's weight in the water, which is how things can keep afloat.

Why do things float better in saltwater?

Whether objects float or sink depends on both the density of the water as well as the object itself.

When salt is added to water, it increases the density of the water.

The salt added increases the mass of the water, which in turn increases the density.

This means that objects that would normally sink in water such as wood, boats, heavy plastic, and other items now have a lighter density and mass than the salty water, enabling them to float on the surface due to the molecules being more spread out.

The saltwater displaced by an added object exerts a higher buoyant force on it, which pushes it upwards and helps it float better.

Test how objects float better in saltwater yourself at home! For this, take two containers of equal size, filled with equal amounts of water.

In one container, add a few tablespoons of salt and make sure it dissolves well. Take an object such as a hollow plastic ball, and drop it into both containers.

You will observe that the ball, when immersed in the saltwater, will float higher than when dropped into the normal water. This is due to the higher density of the saltwater, which exerts a heavy amount of buoyant force on the ball than the freshwater.

Why do denser objects sink?

When an object is placed in water, it has a higher mass than the water it displaces. This is because water is less dense than the object.

This means that the amount of buoyant forces experienced by the object will be less than its mass, as well as the gravitational force acting on it.

Hence the heavy object now sinks in water, as the volume of water displaced is less than the object's weight.

Things made out of high-density materials such as metal, concrete, and stone will most likely sink in water, whereas materials such as cork, wood and paper, which are low in density, will float.

Despite being made of dense materials such as metal and wood, ships float due to the amount of air present inside them due to the hollow hull, which makes them much less dense than water.

Why do things float in space?

On earth we experience a force known as gravity, which ties us to the ground and keeps everything from floating away into the vast expanse of space. The farther you get away from earth, the less gravitational force will affect you.

There is a very small amount of gravity in space, with celestial bodies only being tied to each in orbit due to the gravitational force of each other or larger bodies, such as the sun.

Objects in space are in a constant state of what is known as free fall. In free fall, there is no larger body with a gravitational force (in our case, this is the Earth) for other objects to fall towards.

Due to this, bodies remain in free fall all the time, which makes it look like they are floating. In space, the shape or mass of an object does not matter, it will constantly remain in free fall as there is no air.

For example, if an astronaut on a space station drops a pencil, it will not fall to the ground, but would rather float around as it does not experience any gravitational pull towards anything in particular.

Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for why do things float then why not take a look at where does sugar come from, or why do atoms share electrons in covalent bonds?