## What’s Archimedes Principle?

To be fair I will present three explanations based on three different sources: Original Archimedes Notes, Wikipedia and Encyclopedia Britannica.

### Definition #1 of Archimedes Principle based on Archimedes himself

Any object, totally or partially immersed in a fluid or liquid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

In On Floating Objects, Archimedes suggested (ca. 250 BC)

### Definition #2 of Archimedes Principle based on Wikipedia

Archimedes’ principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. Archimedes’ principle is a law of physics fundamental to fluid mechanics. It was formulated by Archimedes of Syracuse.

Wikipedia.org

### Definition #3 of Archimedes Principle based on Britannica

Archimedes’ principle, physical law of buoyancy, discovered by the ancient Greek mathematician and inventor Archimedes, stating that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body. The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface for an object partially submerged in a liquid.

Britannica.com

## What’s Archimedes principle equation?

There can be different variations of equation dependently on what do we need to calculate.

### Equation #1 – If we would like to calculate how much weight an object lost when placed in a given fluid

Weight of displaced fluid = weight of the object in vacuum – weight of object in fluid

### Equation #2 – If we would like to calculate a buoyancy force

The buoyancy force (Fb) is equal to the weight (Ff) of the fluid that a body in that fluid displaces. The weight Ff can be written in terms of the density (p) of the fluid as Ff = pVg, where V is the volume of the fluid that has been displaced and g is 9.8 metres per second per second, the value of the acceleration from Earth’s gravity.

## What is Archimedes principle example?

The beauty of the Archimedes’ principle is that it works perfectly for most of the fluids and gases. This way you can explain both why ships float and why balloons filled with hot air or helium rise.

If you would like to know how large needs to be a ship to carry only you and nothing else you need to use Archimedes principle. Let’s agree that our ship will float on water and will be made of plastic boxes (from IKEA) .

- Calculate your mass, let’s assume it is 70kg
- Secondly, you need to calculate volume of water that needs to be displaced to equal your mass. V = Ff/pg = mg/pg = 70*9,8/1000*9,8 = 0,07 m3. It is a box having 1m x 1m x 0,07 m.
- Finally, use enough plastic boxes or bottles (closed ones) which total volume is greater than volume of water displaced only by your weight – you want to float in a boat and not sink immediately therefore leave some contingency. Hence, if you build or buy a box which is 1 meter long, 1 meter wide and 20 cm high you are good to float! 🙂 Just be aware that it might flip 😉

We have performed a very attractive experiment Dancing Grapes where Archimedes Principle can be observed in practice and in a very funny way.

## Archimedes principle story

Archimedes was taking a bath when he discovered that his body becomes lighter if it is placed in the water, but this is only a part of the story that made him famous. The more practical usage was that he needed a way to determine if things are made of pure gold or not to act against frauds. During his findings he exclaimed a famous word “Eureka!”. The story goes like this:

King Heron II of Syracuse had a pure gold crown made, but he thought that the crown maker might have tricked him and used some silver. Therefore Heron asked Archimedes to figure out whether the crown was pure gold.

Firstly, Archimedes took one mass of gold and one of silver, both equal in weight to the crown. Secondly, he filled a vessel to the brim with water, put the silver in, and found how much water the silver displaced. Thirdly, he refilled the vessel and put the gold in. The gold displaced less water than the silver. Archimedes put the crown in and found that it displaced more water than the gold and so was mixed with silver.

That Archimedes discovered his principle when he saw the water in his bathtub rise as he got in and that he rushed out naked shouting “Eureka!” (“I have found it!”) is believed to be a later embellishment to the story.

based on Britannica.com

## Archimedes principle experiment

### What is Archimedes experiment about?

In our experiment, we use a dynamometer. We have a bottle with small stones, and we measure its weight first in the air, and next, we immerse the bottle in the water. Finally, we see two different values on our dynamometer. The value is lower in the bottle.

### What skills does the experiment develop?

Firstly, our experiment develops cognitive skills and secondly, it is a great way to introduce basics of physics and ancient scientists. To make the experiment more attractive for kids, we create a “sea world” with fish and shells in the blue water.

### What do we need to perform this experiment?

- Vase
- Water
- Food coloring
- Stickers
- Dynamometer
- Bottle with stones or another object

## Final thoughts

Please let me know in the comments below if you liked my experiment and if your kids liked it. Write a comment what is your way to introduce physics to your kids?

Send me an email what experiment would you like to see?