Anyone who has been to a swimming pool or to the beach is familiar with the idea that some things float, while other things sink. The question though is why do some things float, and other things sink.

Is it their weight?

Consider that a very large ship easily floats on the ocean while a very small pebble sinks. The ship is much heavier than the pebble. Yet the lighter pebble sinks while the very heavy ship floats. So what is really happening?

The answer to this question has to do with buoyancy. Buoyance is a science term that simply refers to how likely something is to float or sink. Things with a positive buoyancy will float, while things with a negative buoyancy will sink. If something has a neutral buoyancy then it will neither sink nor float. Instead it will simply move around with the fluid it is in and follow its currents.

Let’s talk for a minute about what a fluid is. In science, a fluid is any material that flows. This can be a liquid or a gas. You might not think of air as a fluid because it doesn’t really seem to flow like water does. But air is actually a fluid. You can see this for yourself the next time you are blowing bubbles. The bubble will change shape as it floats through the air. If air wasn’t a fluid, then the bubble would keep its shape and not flow like it does.

So what causes buoyancy?

The answer to this has to do with the pressure of the fluid on an object. When an object is placed in a fluid, the fluid will exert a pressure on the object. This pressure will be equal in all directions. This is called hydrostatic pressure.

The pressure on an object submerged in a fluid is caused by the weight of the fluid above the object. The more fluid there is above the object, the greater the pressure will be.

This hydrostatic pressure acts on the object in all directions. So if you have an object that is completely submerged in a fluid, the pressure will be equal on all sides of the object.

The pressure on an object floating in a fluid is different. The pressure on the bottom of the object is greater than the pressure on the top of the object. This is because the weight of the fluid is pressing down on the object from above.

You have likely experienced this for yourself when swimming in a swimming pool. Have you noticed that the deeper you dive, the more pressure you feel? Your ears are especially sensitive to these changes in pressure. Thus the deeper you go the more your ears might hurt as the pressure increases.

The difference in pressure on the top and bottom of an object is what causes the object to float. The pressure on the bottom of the object is greater than the pressure on the top of the object. This imbalance in pressure creates a force on the object that pushes it up into the fluid.

This force is called upthrust. Upthrust is the force that opposes the force of gravity and causes an object to float.

The amount of upthrust an object experiences is related to its density. The less-dense an object is, the greater the upthrust it will experience.

The density of an object is simply how much mass the object has compared to its volume. An object with a lot of mass and a small volume will have a high density. An object with a small amount of mass and a large volume will have a low density.

The denser an object is, the more it will sink in a fluid. The less-dense an object is, the more it will float in a fluid.

This is why a very large ship can float on the ocean. The ship has a lot of mass but it also has a large volume. This gives it a low density. The low density of the ship causes it to float in the water.

The same is true for a pebble. A pebble has a lot of mass but a small volume. This gives it a high density. The high density of the pebble causes it to sink in water.

The same is true for you! You have a lot of mass but a relatively small volume. This gives you a high density. The high density of your body causes you to sink in water.

But why do you float in water and not sink to the bottom? The answer has to do with the amount of air in your lungs. When you take a deep breath, your lungs fill up with air. This air makes your body less dense than it would be if you were holding your breath. The less-dense body causes you to float in water.

Have you ever wondered why a helium-filled balloon floats? Helium is a gas with a very low density. The low density of the helium makes the balloon less dense than the air around it. This causes the balloon to float in the air.

You might also be wondering why a lead pipe sinks in water but a Styrofoam cup floats. The lead pipe is very dense, while the Styrofoam cup is very light. The lead pipe sinks because its density is greater than the density of water. The Styrofoam cup floats because its density is less than the density of water.

The first person to study buoyancy was a man who lived in ancient Greece by the name of Archimedes. Archimedes discovered that the amount of upthrust on an object is equal to the weight of the fluid that the object displaces.

This principle is known as Archimedes’ principle. It states that the upward force on an object floating in a fluid is equal to the weight of the fluid that the object displaces.

The displaced fluid is the volume of fluid that the object takes up. The displaced fluid is equal to the volume of the object.

The displaced fluid has weight because it has mass. The volume of the displaced fluid is equal to the volume of the object. However, the mass will be different. If the fluid has a greater mass per unit of volume than the object will float. If the object has a greater mass per unit of volume than the object will sink. Put another way, if the object is more dense than the fluid it will float, but if it is less dense than the fluid it will sink.

In science this is referred to as the Archimedes Principle.