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.

Archimedes' principle

Main article: Archimedes' principle

Archimedes' principle explains why things float or sink. It is named after Archimedes, who found it a long time ago in 212 BC. This principle works for objects in liquids and gases. It says that when you put an object into a liquid or gas, it pushes away some of that liquid or gas. The force pushing the object up is the same as the weight of the liquid or gas the object pushes away.

If an object is fully under the liquid, it pushes away liquid equal to its own size. If the object floats, the weight of the liquid it pushes 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.

Forces and equilibrium

The pressure inside a fluid gets bigger as you go deeper because of the weight of the fluid above. This means the pressure at the bottom of a column of fluid is more than at the top.

The difference in pressure between the top and bottom of an object causes an upward force called buoyancy. This force is the same as the weight of the fluid pushed away by the object. If the buoyancy force is bigger than the object's weight, it will rise; if it is smaller, the object will sink. When floating, only the part of the object under the water pushes away fluid.

States of buoyancy

Buoyancy explains how things act when you put them in a liquid like water. There are three ways buoyancy can work.

If the push up from the water is stronger than the object's weight, the object will float up. This is called positive buoyancy.

If the push up from the water is just the same as the object's weight, the object will stay still. This is called neutral buoyancy. See: Neutral buoyancy

If the push up from the water is weaker than the object's weight, the object will sink down. This is called negative buoyancy.

Compressible objects

When an object moves up or down in water or air, the pressure around it changes. Because objects can be squeezed a little, their size can change too. This affects how much they float.

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.

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 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 is lighter than the water it pushes aside, 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 the same 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.