Do Tennis Balls Float? The Physics Behind It

Do Tennis Balls Float?

It’s a strange question that some fans seem to be asking us but we are happy to research and answer this – do tennis balls float in water?

The best way to answer the question is by testing it yourself.

Take a tennis ball and throw it in a bucket of water. Does it sink or does it float? You would most likely see that it begins to float, however, depending on the type of tennis ball, there may be some that sink.

So why does it float or sink? To answer this question we should first look at reasons as to why objects float or sink? First things first though…

Do Tennis Balls Float?

To understand this let’s see how a tennis ball is made.

The core of the ball is made of rubber. There are two rubber spheres which are glued together. Thus a tennis ball is hollow and filled with either a gas or air.

This creates pressure inside the rubber core and helps the ball to bounce better. But rubber balls can wear out soon, so they are covered with felt, which increases the durability of the balls.

The type of ball discussed above is a typical tennis ball, known as pressurized balls.

Some balls are however non pressurized, also known as pressureless balls. These don’t have hollow core, instead the core is solid and it’s made of hard rubber. These balls however don’t feel lively and have lower bounce than pressurized balls.

So, the more common pressurized balls which are hollow and filled with air are less dense than water and therefore, they float.

The less common pressure-less balls are not hollow, instead they are solid, which makes them denser than water and hence if they are placed in water they sink.

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What Makes an Object Float?

Take a piece of stone and drop it in a bucket of water and you would see that it goes right at the bottom and settles there.

Now let’s take a tennis ball or a piece of wood and drop it in water, we would see that they don’t sink but instead start floating. Even if you try to push them down, they shoots up and start to float again.

So why did the stone sink and the piece of wood did not? To understand we must first learn the concept of density.


All objects are made up of molecules. While in some objects, molecules are tightly packed; in others, the molecules are loosely packed. Objects in which the molecules are tightly packed are compact and more dense than objects that have loosely packed molecules.

If the density of an object is more than the density of water then it sinks, however, if the density of an object is less than the density of water, then it starts to float.

Now why does that happen? That’s because when you drop anything in water or any liquid, the liquid applies an upward force on the object which is known as buoyant force.

So if the liquid is denser than the object then the buoyant force is more than the force of gravity as a result of which the object will float. In contrast, if the object has more density than the liquid, then the buoyant force is lesser than the force of gravity and so the object sinks.

Density can be explained as the relationship between the mass of the substance and the volume it takes up. It is calculated by dividing mass of an object with its volume.

The principle of density was given by Greek scientist Archimedes.

The Archimedes Principle

The Archimedes Principle states that “the upward buoyant force that is exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.”

We will take an in depth look at the definition in the subsequent paras, but first let’s understand how Archimedes discovered this principle.

King Heiron II of Syracuse had ordered a pure gold crown, but once he received the crown he doubted if it was pure gold. He was suspicious that the crown-maker had added some silver to the crown. The king called upon Archimedes to find out if the crown was made of pure gold.

The challenge for Archimedes was to get to the bottom of the truth, while also protecting the crown from even a scratch.

One day, while taking bath he discovered his principle. He saw the water in the bathtub rose as he got inside. The deeper he went the more the water rose. Excited by the discovery, he is said to have ran out naked shouting “Eureka” (meaning I have found it!).

Archimedes then took one mass of gold and one of silver, both equal in weight to the crown.

First he put the silver in a vessel full of water and made note of how much water was displaced. Then he refilled the vessel and put the gold in. He saw that gold displaced less water than the silver.

He then refilled the vessel and put the crown in. It displaced more water than gold and thus it was clear that the crown was a mix of gold and silver.

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Relationship Between Buoyant Force and Weight of Liquid Displaced

One may have realized that while swimming the body feels lighter than on earth. That’s because when an object is put in water the buoyant force acts against the gravitational force and thus the object loses some weight.

The upward thrust or buoyant force of the liquid is equal to the weight of the water displaced by the object.

Now, if the weight of the water displaced is equal to the weight of the object then buoyant force is equal to the gravitational force and thus the object floats.

The object will also float if the weight of the water displaced is greater than the weight of the floating body. However, if the weight of the water displaced is less than the weight of the object. Then the buoyant force is lesser than the gravitational force and hence the object sinks.

Why Ships Float?

Ships are made of steel, which is denser than water, so they should sink, but they don’t. The reason is the shape of the ship. A ship is hollow and the empty space within the ship contains air due to which the average density of the ship is less than that of water and hence it floats.

At times when ships sink, it happens because water somehow manages to get into the ship. Water replaces the air in the hollow body and thus makes the density of ship more than that of water, thus causing the ship to sink.

So now, we know why objects float or sink in water, so let’s try to reason why a tennis ball will float and not sink.

Final Word on Floating Tennis Balls

Tennis balls are most commonly made of two rubber halves, which are combined together and covered  with fabric.

They are hollow inside and filled with air or gas, which create pressure inside, hence known as pressurized balls. Because they are hollow inside, they are less dense than water and hence they float.

In contrast, non-pressurized balls are solid inside. This makes them denser than water and hence they sink.

Gaurav Banerjee

Needless to say, a tennis fan who would like to travel the world watching this glorious sport. Also, a writer on

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