Buoyancy

Buoyancy is the upward force that a liquid or gas pushes on an object placed in it. This force works against the weight of the object. Imagine you push your hand into a pool of water. You feel the water pushing back up on your hand. That push is buoyancy.

The strength of this upward push depends on how deep the object is in the liquid and how much liquid the object pushes aside, or displaces. If an object is lighter than the amount of water it pushes aside, it will float. If it is heavier, it will sink. This idea is known as Archimedes' principle.

Buoyancy is very important in many natural processes. It helps explain why warm air rises and cool air sinks, creating weather patterns. It also plays a role in how boats stay on top of water and how oil and water separate from each other. Buoyancy needs a force like gravity or acceleration to work, and it can even happen in places without gravity, as long as there is some kind of push or pull acting on the objects.

Archimedes' principle

Main article: Archimedes' principle

Archimedes' principle tells us why things float or sink. It is named after Archimedes, who discovered it a long time ago in 212 BC. This principle works for objects in liquids and gases. It says that any object put into a liquid or gas will push away some of that liquid or gas. The force pushing the object up is equal to the weight of the liquid or gas that the object pushes away.

If an object is completely under the liquid, the amount of liquid pushed away is the same as the size of the object. If the object floats, the weight of the liquid pushed away is the same as the weight of the object. This is why wood floats on water—it is lighter than the water it pushes away.

Applications

One important use of Archimedes' principle is called hydrostatic weighing. When an object is placed in a liquid and the forces balance out, we can find out how dense the object is compared to the liquid. This helps us measure density without needing to know the object's size.

Forces and equilibrium

The pressure inside a fluid increases with depth because of the weight of the fluid above. This means the pressure at the bottom of a column of fluid is greater than at the top.

The difference in pressure between the top and bottom of an object causes an upward force called buoyancy. This force equals the weight of the fluid displaced by the object. If the buoyancy force is greater than the object's weight, it will rise; if it is less, the object will sink. When floating, only the submerged part of the object displaces fluid.

States of buoyancy

Buoyancy describes how objects behave when placed in a fluid like water. There are three possible states of buoyancy.

If the upward force from the fluid is greater than the object's weight, the object will float upwards. This is called positive buoyancy.

If the upward force exactly matches the object's weight, the object will stay in one place. This is called neutral buoyancy. See: Neutral buoyancy

If the upward force is less than the object's weight, the object will sink downwards. This is called negative buoyancy.

Compressible objects

When an object moves up or down in a fluid, the pressure around it changes. Because all objects can be squeezed a little, their size can change too. This affects how much they float. If the object is squeezed more than the fluid around it, it can become unstable and move up or down easily.

Submarines dive by letting water into special tanks. When these tanks are filled, the submarine becomes heavier and can sink. By adjusting these tanks, sailors can make the submarine stay at one depth without moving up or down. Balloons float higher when the air around them gets thinner, but they stop rising when they balance with the air around them.

Divers under water often need to adjust their floating ability. They wear special gear with air pockets to help them stay at the right depth. As they breathe out or use up air, they need to make small changes to keep from floating up or sinking down.

Density

If an object weighs less than the water it pushes aside when fully under the water, it will float. This happens because the water pushes the object up with a force bigger than the object’s weight. This is called positive buoyancy. If the object is completely under the water, like a submarine or air inside a balloon, it will move upward.

If the object has the same average weight as the water around it, it will stay in one place without sinking or floating. This is called neutral buoyancy. But if the object is heavier than the water it pushes aside, it will sink. This is called negative buoyancy. Even heavy things like steel ships can float because they hold lots of air inside, making their overall weight less than the water they push aside.