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How to Make Something Float Using Density

Sarah Calderado, David Wood
  • Author
    Sarah Calderado

    Sarah Calderado has taught middle and high school science for over 7 years. They have a bachelors degree in geoscience and a masters degree in secondary education from University of North Carolina Wilmington. They also have a North Carolina teaching license..

  • Instructor
    David Wood

    David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn how to make something float on water. Understand what determines if an object sinks or floats and examine how an object's density affects floatation ability. Updated: 01/30/2022

What Determines if an Object Sinks or Floats?

Why does a wooden cube float when placed in a glass of water, but a copper cube that is the same size as the wooden cube sinks? Every object on the planet is subjected to the force of gravity pulling the object towards the earth. Gravitational force pulls an object in a downward direction towards the earth. When a person places a wooden cube or a copper cube in a glass of water, the cube will be pulled down by gravity towards the bottom of the glass. So, what is stopping the wooden cube from being pulled to the bottom of the glass, yet allowing the copper cube to be pulled through the water and towards the bottom of the glass by gravity?

Buoyancy

The answer is buoyancy. Buoyant force is the force exerted by a fluid on an object in the opposite direction of gravitational force and equal to the volume of the fluid displaced. It is as if gravity is pulling the object down through the water, but all of the water molecules in the glass are pushing the object up and away from the bottom of the glass.


Buoyant force (FA) acts in opposition to gravitational force (Fp), resulting in the buoyancy (Vi) of an object.

Buoyancy


Archimedes Principle

The Greek mathematician Archimedes explains this phenomenon in what is now called Archimedes Principle. When an object is placed in a fluid, that fluid will exert a force back on the object that is opposite the direction of gravity. This is similar to two people pushing back and forth against each other. If one person is stronger than the other, they will be able to push the other person away. If the buoyant force is stronger than gravity, the object will be pushed away from the bottom of the glass and float. If the buoyant force is weaker than gravity, the object will sink. When placed in a glass of water:

  • Wood will float because the buoyant force acting on it is stronger than the force of gravity.
  • Copper will sink because the buoyant force acting on it is weaker than the force of gravity.

Buoyancy and Forces

Some objects float. Some objects sink. If you jump into some water, you'll probably sink. But if you spread your body out and lay back, you'll likely float. If you throw a pool noodle into the pool, it will float. But a brick... not so much. But not all objects are obvious. How do we figure out if an object will float or sink?

To do that, we need to talk about buoyancy and forces. If an object is put into water, there are two forces acting on it: gravity and the buoyant force.

Gravity's always the same, whether you're inside the water or outside of water. The force of gravity is equal to the mass multiplied by the acceleration due to gravity, which is 9.8 on the surface of the Earth. So if an object is 10 kilograms in mass, the force of gravity will be approximately 98 newtons, whether it's in water or on dry land.

So whether or not an object floats has a lot more to do with the buoyant force. Buoyancy is the ability or tendency of an object to float in a fluid: a liquid or a gas. This happens because fluid pressure increases with depth. As you go underwater, diving down deeper, the molecules are closer together. Because of this, they hit you more. So the pressure gets bigger the deeper you go. If you put an object underwater, there will be more pressure on the bottom of the object than the top, because the bottom is deeper underwater. And this creates an upwards force.

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Relationship between Density and Floating

Every object placed in water will sink or float depending on the buoyant force acting on the object in opposition to gravity. If this is true, why do objects that are the same size, like the wooden and copper cubes, have seemingly different buoyant forces acting against gravity? The answer is density. Density is the measure of the mass of an object in relation to its volume. The greater mass an object has within the same amount of volume, the greater its density. Every object on earth has a density that can be calculated, including liquid water.

  • The density of liquid water is generally cited as {eq}1 gram/ cm^3 {/eq}. Some common conversions are {eq}1 kilogram/ m^3 {/eq}, and {eq}62.4 pounds/ ft^3 {/eq}.

If the object has a greater density than water, it will overcome the buoyant force acting upon it and sink. If it has a lesser density than water, it will be weaker than the buoyant force acting upon it and float.

A larger object with a density less than {eq}1 gram/ cm^3 {/eq} will float on top of the water, because the buoyant force exerted by the water {eq}1 gram/ cm^3 {/eq} is stronger than the pull of gravity on the object. A smaller object with a density of more than {eq}1 gram/ cm^3 {/eq} will sink, because the buoyant force exerted by the water {eq}1 gram/ cm^3 {/eq} is weaker than the pull of gravity on the object.

Formula for Calculating Density

Examine the wooden and copper cubes more closely. If each cube is 1 centimeter (cm) long, 1 centimeter (cm) tall, and 1 centimeter (cm) wide, its resulting volume will be equal to {eq}1 gram/ cm^3 {/eq}. The difference is, that each cube does not have the same amount of mass within the cube.

  • Mass of wooden cube = 0.45 grams
  • Mass of copper cube= 8.96 grams

To calculate density, we use the equation:

{eq}Density=\frac{Mass}{Volume} {/eq}

Will It Float or Sink?

Okay, but how do we know if an object will float or sink? Well, according to Archimedes Principle, the buoyant force on a submerged object is equal to the weight of the liquid displaced by the object. Which means that the larger the amount of an object that is underwater, the stronger the buoyant force will be.

A really large, light object is probably going to float. This is because a large object will displace a lot of water, creating a big buoyant force. And a light object will have a small force of gravity. With a small force of gravity and a big buoyant force, the object will definitely float.

But a really small, heavy object is probably going to sink. This is because a small object will not displace much water and will have a much smaller buoyant force. And, being heavy, it will also have a large force of gravity.

The combination of size and mass is called density. Density is a measure of how compact the mass in a substance or object is. Or in other words, density is mass spread out over a volume. The more each meter cubed weighs, the more dense the substance is. Heavy, small objects are super dense. And large, light objects are not very dense at all.

It turns out that density is really useful for figuring out if an object will sink or float. An object that is more dense than water will sink. And an object that is less dense than water will float.

Water has a density of 1000 kilograms per meters cubed. So all we have to do to figure out if something will float or sink is take some measurements, calculate the object's density and compare it to 1000.

The density of an object or substance can be calculated from this equation: density, in kilograms per meter cubed, is equal to mass, in kilograms, divided by volume, in meters cubed. So if you weigh an object to get its mass, measure an object to calculate its volume (its size) and divide the mass by the volume, you will be able to answer the question.

Example

Let's go through an example.

An object shaped like a cube is weighed and turns out to have a mass of 3 kilograms. You measure one side of the cube and it's 0.2 meters long. Will the object float or sink?

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Video Transcript

Buoyancy and Forces

Some objects float. Some objects sink. If you jump into some water, you'll probably sink. But if you spread your body out and lay back, you'll likely float. If you throw a pool noodle into the pool, it will float. But a brick... not so much. But not all objects are obvious. How do we figure out if an object will float or sink?

To do that, we need to talk about buoyancy and forces. If an object is put into water, there are two forces acting on it: gravity and the buoyant force.

Gravity's always the same, whether you're inside the water or outside of water. The force of gravity is equal to the mass multiplied by the acceleration due to gravity, which is 9.8 on the surface of the Earth. So if an object is 10 kilograms in mass, the force of gravity will be approximately 98 newtons, whether it's in water or on dry land.

So whether or not an object floats has a lot more to do with the buoyant force. Buoyancy is the ability or tendency of an object to float in a fluid: a liquid or a gas. This happens because fluid pressure increases with depth. As you go underwater, diving down deeper, the molecules are closer together. Because of this, they hit you more. So the pressure gets bigger the deeper you go. If you put an object underwater, there will be more pressure on the bottom of the object than the top, because the bottom is deeper underwater. And this creates an upwards force.

Will It Float or Sink?

Okay, but how do we know if an object will float or sink? Well, according to Archimedes Principle, the buoyant force on a submerged object is equal to the weight of the liquid displaced by the object. Which means that the larger the amount of an object that is underwater, the stronger the buoyant force will be.

A really large, light object is probably going to float. This is because a large object will displace a lot of water, creating a big buoyant force. And a light object will have a small force of gravity. With a small force of gravity and a big buoyant force, the object will definitely float.

But a really small, heavy object is probably going to sink. This is because a small object will not displace much water and will have a much smaller buoyant force. And, being heavy, it will also have a large force of gravity.

The combination of size and mass is called density. Density is a measure of how compact the mass in a substance or object is. Or in other words, density is mass spread out over a volume. The more each meter cubed weighs, the more dense the substance is. Heavy, small objects are super dense. And large, light objects are not very dense at all.

It turns out that density is really useful for figuring out if an object will sink or float. An object that is more dense than water will sink. And an object that is less dense than water will float.

Water has a density of 1000 kilograms per meters cubed. So all we have to do to figure out if something will float or sink is take some measurements, calculate the object's density and compare it to 1000.

The density of an object or substance can be calculated from this equation: density, in kilograms per meter cubed, is equal to mass, in kilograms, divided by volume, in meters cubed. So if you weigh an object to get its mass, measure an object to calculate its volume (its size) and divide the mass by the volume, you will be able to answer the question.

Example

Let's go through an example.

An object shaped like a cube is weighed and turns out to have a mass of 3 kilograms. You measure one side of the cube and it's 0.2 meters long. Will the object float or sink?

To unlock this lesson you must be a Study.com Member.
Create your account

Frequently Asked Questions

Why do higher density objects sink?

The density of water is relatively low at 1 gram per centimeter cubed. Higher density objects usually have a density greater than water. When a high density object is placed in water, the gravitational pull on the object due to its mass is greater than the buoyant force of the displaced water acting against it. When the gravitational pull is greater than the buoyant force, the object will sink.

What makes things sink or float?

An object will sink or float depending on its density compared to the density of the fluid that it is placed in. If the object is more dense than the fluid, it will sink. If the object is less dense than the fluid, it will float.

How do you make an object float?

In order to make an object float, its volume in relation to its mass must be increased. To do this, change the shape of the object such as flattening the object or stretching the object into a cup shape in order to create more volume without adding mass. Once the density of the object is less than the density of water, it will float.

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