Archimedes’ Principle: The Simple Floating Beaker Experiment That Makes Upthrust Visible

Some physics experiments are impressive because they use expensive equipment, flashing sensors or complicated data logging. Others are powerful because they are wonderfully simple.

Archimedes’ Principle is one of those ideas that can feel abstract when written as a sentence in a textbook:

The upthrust on an object in a fluid is equal to the weight of the fluid displaced.

Students often learn the words. They may even be able to quote the definition in an exam. But do they really understand what it means?

That is where a large measuring cylinder, some water, and a small floating beaker can do something remarkable. They can make upthrust visible.

The Experiment: A Beaker Floating Inside a Measuring Cylinder

The setup is very simple.

Take a large measuring cylinder or tall transparent container and partly fill it with water. Then place a small empty beaker, plastic cup, or weighing boat into the water so that it floats.

At first, the beaker floats high in the water. Only a small part of it is submerged because the beaker is light. It does not need to displace much water to support its own weight.

Now slowly add water into the floating beaker.

The beaker sinks lower and lower.

It is not sinking because it is “failing” to float. It is still floating. It is simply becoming heavier, so it must displace more water to produce a larger upthrust.

This is the key point.

A floating object adjusts how much water it displaces until the upthrust equals its weight.

The Important Detail: Where the Added Water Comes From

There is one subtle but important teaching point here.

If you add water to the floating beaker from outside the measuring cylinder, then you have added extra water to the whole system. The water level in the measuring cylinder will rise.

However, if the water poured into the floating beaker is taken from the surrounding water in the same measuring cylinder, the overall water level remains the same.

This is the clever part.

The water has simply been moved from outside the beaker to inside the beaker. The floating beaker now displaces extra water equal to the weight of the water placed inside it. That extra displacement balances the volume of water removed from the surrounding cylinder.

So the beaker floats lower, but the water level does not change.

To many students, this feels surprising at first. It looks as though the beaker should make the water rise as it sinks lower. But Archimedes’ Principle explains exactly why it does not.

What Is Upthrust?

Upthrust is the upward force exerted by a fluid on an object.

When an object is placed in water, it pushes water out of the way. The water pushes back. This upward push is upthrust.

For a floating object:

Upthrust = weight of the object

For Archimedes’ Principle:

Upthrust = weight of fluid displaced

So for a floating beaker:

Weight of beaker + weight of water inside it = weight of water displaced

The beaker sinks lower until enough water has been displaced to balance the total weight.

This is why a small empty boat floats high, but a heavily loaded boat floats lower. It is also why there is a load line painted on ships. The ship is allowed to sit lower when carrying cargo, but only up to a safe limit.

Why This Experiment Works So Well for A Level Physics

At A Level, students are expected to move beyond simply saying “things float because they are less dense than water.”

That explanation is often incomplete.

A steel ship floats, but steel is denser than water. A hollow metal can may float, but a solid piece of the same metal may sink. A plastic beaker floats differently depending on whether it is empty or full.

The real explanation involves forces, density, displaced volume and equilibrium.

This simple experiment allows students to see:

• An object floating in equilibrium
• Weight acting downwards
• Upthrust acting upwards
• Greater weight requiring greater displacement
• The connection between volume displaced and force
• Why floating objects sit lower when loaded

It also shows why physics is not just a collection of equations. The equation describes something real that can be observed on the bench.

A Simple Calculation to Support the Demonstration

Suppose the empty beaker has a mass of 50 g.

Its weight is approximately:

0.05 kg × 9.8 N/kg = 0.49 N

To float, it must displace water weighing 0.49 N.

Since water has a density of about 1000 kg/m³, the beaker needs to displace about 50 cm³ of water.

Now add 100 g of water to the beaker.

The total mass becomes:

50 g + 100 g = 150 g

The total weight is now approximately:

0.15 kg × 9.8 N/kg = 1.47 N

The beaker must now displace about 150 cm³ of water.

So it floats lower.

The beaker has not become “less floaty.” It has simply become heavier and therefore needs to displace more water to remain in equilibrium.

The Bucket and Pail Version

Another classic version of this experiment is the bucket and pail demonstration.

An object is lowered into water and the displaced water is collected in a small pail or overflow can. The weight of the displaced water can then be compared with the upthrust on the object.

This version is excellent because it makes the phrase “weight of water displaced” feel more physical. The displaced water can be collected, measured and weighed.

The bucket and pail method is particularly useful when teaching students who need to connect the idea of upthrust with practical measurement. It also links naturally to required practical skills: accurate measurement, uncertainty, repeats and experimental design.

However, the floating beaker version has a different advantage. It is extremely visual. Students can see the beaker sinking lower as more water is placed inside it. There is no need for a complex apparatus. The physics is right there in front of them.

Common Misconceptions This Demonstration Helps Fix

One of the most common misconceptions is that floating depends only on density.

Density is important, but it is not the whole story. Shape and displaced volume matter. A lump of metal may sink, but a metal boat can float because it encloses air and displaces a large volume of water.

Another misconception is that the upthrust is always equal to the weight of the object. That is only true when the object is floating or stationary in equilibrium. If an object is sinking, the weight is greater than the upthrust. If it is rising, the upthrust is greater than the weight.

A third misconception is that objects float “because water pushes them up.” That is partly true, but students need to go further. They need to explain how the size of that upward force is determined.

Archimedes’ Principle gives the complete explanation.

Bringing It Back to Real Life

This experiment is not just about a beaker in a measuring cylinder. It explains some very real situations.

It explains why a rowing boat sits lower when more people climb into it.

It explains why cargo ships have load lines.

It explains why a submarine can rise or sink by changing the amount of water in its ballast tanks.

It explains why life jackets work by increasing the volume of water displaced without adding much weight.

It explains why a person may float more easily in seawater than freshwater, because seawater is denser and provides a greater upthrust for the same displaced volume.

These real-world examples help students see that Archimedes’ Principle is not an isolated classroom trick. It is one of the key ideas behind floating, sinking, ship design, submarines, hydrometers and even swimming.

A Personal Reflection: Simple Experiments Often Teach Best

I have always liked experiments like this because they remind students that physics does not always need to begin with algebra.

Sometimes it begins with noticing something odd.

A beaker floats. Add water and it floats lower. Take the added water from the surrounding container and the level stays the same. That is a puzzle.

Once students have seen the puzzle, the equation has a reason to exist.

This is especially important at A Level. Students can become very good at rearranging equations while still not fully understanding the physical situation. A simple demonstration forces them to ask what is actually happening.

The best practical work is not always the most complicated. Sometimes the most effective lesson comes from a beaker, a cylinder and a question:

Why did the water level not change?

Teaching Extension: Turning It Into an A Level Discussion

For stronger students, this experiment can be extended into a deeper discussion.

Ask them to predict what will happen before the water is added. Then ask them to explain the result using forces. Then ask them to explain it using density and displaced volume. Finally, ask them to write a short exam-style answer using correct scientific language.

A good answer might include:

As water is added to the floating beaker, the total weight of the beaker increases. The beaker therefore sinks lower into the water so that it displaces a greater volume of water. The increased volume of displaced water produces a larger upthrust. When floating in equilibrium, the upthrust is equal to the total weight of the beaker and the water inside it.

That is exactly the kind of explanation examiners want: clear, precise and linked to the correct principle.

Conclusion: Archimedes Made Visible

Archimedes’ Principle is one of the great ideas in physics because it connects forces, fluids, density and equilibrium in one elegant statement.

But for students, it can remain just a phrase unless they see it happen.

The floating beaker experiment makes the principle visible. The beaker sinks lower as it becomes heavier. It displaces more water. The upthrust increases. The object continues to float when the forces are balanced.

It is simple, visual and memorable.

And sometimes that is exactly what good physics teaching needs.