Archimedes' principle

Archimedes' principle tells us about how things behave in water or other liquids. It says that when you put an object in a liquid, the liquid pushes back up on the object with a force. This upward push is called the buoyant force.

The size of this force is exactly the same as the weight of the liquid that the object pushes out of the way, or displaces.

This idea is very important in physics. It helps us understand why some things float and others sink. The principle was discovered by a smart man named Archimedes who lived in a place called Syracuse a long time ago. His work helps engineers and scientists today when they build ships, boats, and many other things that need to stay up in water.

Explanation

In his book On Floating Bodies, Archimedes explained that when something is placed in water or any other liquid, it pushes up with a force. This force is equal to the weight of the water it pushes aside. This is called Archimedes' principle.

This principle helps us understand why some things float and others sink. If the upward push is stronger than the object's weight, it will float. If the push is weaker, the object will sink. If the forces are just right, the object will stay in one place.

Formula

Archimedes' principle explains why things float or sink in water. When you put an object in water, the water pushes back up on the object. This upward push is called the buoyant force. The buoyant force equals the weight of the water that the object pushes aside or displaces.

For example, imagine a rock that weighs 10 newtons in air. When you place it in water, it pushes aside water that weighs 3 newtons. The rock then feels like it only weighs 7 newtons because of the water's upward push. This makes it easier to lift the rock in water than in air. If an object’s weight is less than the weight of the water it pushes aside, it will float. If it’s heavier, it will sink.

Forces and equilibrium

The pressure inside a fluid gets stronger the deeper you go. This is because more fluid above pushes down harder. This pressure makes an upward force called buoyancy.

When you put an object in a fluid, it feels this upward force. The force is as strong as the weight of the fluid the object pushes aside. If the object’s weight is less than this force, it will float. If it is more, it will sink. This helps explain why some things float in water and others sink.

Refinements

Archimedes' principle does not include a force called surface tension. This force can act on an object in a fluid.

The principle also does not work perfectly with very complicated fluids.

There is a special case where Archimedes' principle might not work exactly. This happens when one side of an object touches the bottom or side of its container, and no fluid gets under that side. In this case, the upward force might be different from what Archimedes' principle predicts, because the pressure is not the same on all sides.

Principle of flotation

Archimedes' principle helps us understand why objects float. It says that when an object floats, it pushes away a weight of water or air equal to its own weight.

For example, a boat floats because it pushes away enough water to match its weight. If the boat gets too heavy, it will sink.

Think of a heavy block of iron. On its own, it will sink because it doesn’t push away enough water. But if we shape that same iron into a bowl, it can float. The wider and deeper the bowl is in the water, the more water it pushes away. When it pushes away enough water, it can float. This is how ships and submarines are designed—to push away enough water or air to stay up.

Eureka

Main article: Eureka (word)

Archimedes is said to have shouted "Eureka" when he found a way to tell if a crown was made of fake gold. He didn’t use what we now call Archimedes’ principle in the story, but there’s another way using that idea. If you weigh the crown and a piece of pure gold in the air and then put them in water, the balance will change if the crown isn’t as heavy as pure gold. This happens because of how much water the crown pushes aside.