Things That Float
DENSITY
Materials
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Empty 2 Liter Bottle
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Pipet
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Hex Nut
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Water
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A container with water to test how the pipet floats
Part 1:
The Cartesian Diver
This experiment seems nothing short of magical. But it isn't magic, it's science! The Diver is named after the French philosopher, mathematician and scientist René Descartes who is said to have invented it to illustrate buoyancy and and gas laws. When I lightly squeeze the side of the bottle, water is forced up into the pipet, increasing the overall density and making the pipet sink. When I release, it floats back up to the top. Let's take a closer look at the Diver and Density.
What's a Cartesian Diver?
Building Instructions
Follow these steps to create your own Cartesian Diver:
1. Place the Hex Nut on the pipet and push up until it is snug. It should be fairly tight and not fall if you move it around.
2. Cut off excess plastic from the tip leaving 1 cm below the bolt.
3. Next, fill your pipet about 1/2 full of water. Place it in a container filled with water to make sure that it barely floats. The more it floats, the harder that you will have to squeeze to make it sink. You want it to sink easily. This part may take some fiddling around.
4. Fill your bottle up almost all the way to the top with water.
5. Place the pipet in the water.
6. Top off the bottle until the water reaches the edge of the bottle.
7. Seal the bottle with the cap.
8. Squeeze lightly on the sides, making sure that the pipet sinks. If it doesn't sink easily, you may have to take the pipet out and add more water.
More instructionalhelp
Before we can understand how the Cartesian Diver works, we need to understand
DENSITY
In short, density is how much matter is in how much space. It is the mass per unit of volume or how many molecules in a unit of space. Most of the time, the less dense an object is, the more likely it is to float. AS you can see in the illustration to the right, the less dense that an object is, the more spread out the molecules within it are. It's like being in a room with people that you either don't care for or don't know very well. In the more dense object, the molecules are packed together closely. Think of it like being at a family reunion and your great aunts are crowding around you and asking you too many questions.
How and why does the Cartesian Diver work?
When I squeeze the bottle, it increases the pressure in the bottle and forces water up into the dropper. This increases the mass and the density, causing the pipet to sink. When I release my hand, water is forced back out and air is allowed back into the dropper causing it to float back to the surface.
In the picture to the left, in the water you can see two squares (one floating and one at the bottom) and two circles (one floating and one at the bottom). You can see that the size of the object is not related to its density. The squares are the same size, yet one sinks and one floats. The floating square is made out of wood- a material that is less dense than the water. The sunken square is made out of metal- a material that is more dense than water. The balls are different sizes. The large one is more buoyant and floats. The metal ball is more dense and sinks. Lets explore BUOYANCY further.
When the smaller metal ball is placed in the water, it displaces or pushes away the water around it. According the Archimedes Principle, any object that is submerged in a fluid is kept afloat by a force equal to the weight of the fluid displaced by it. The larger ball is kept afloat because the force of the water that it is displacing is more than the gravitational force that is pulling it down. However, the density and gravitational pull of the smaller ball are greater than that of the water displaced, so it sinks. The ball (or any object) will float if the ball can displace an amount of water that is equal to or greater than its weight.
As you can see in the picture with the balls and the square, there is also a piece of sunken clay. Hmmm. Why does the clay sink when it is balled up, but float when it is in the shape of a boat? The weight of the clay doesn't change. The answer is in displacement. Changing the shape of the clay doesn't change the weight, but changes how much water the clay will displace. The more water an object can displace, the more the water can push up. This push is called a buoyant force. The clay boat was able to displace enough water so that the weight of the water was greater than or equal to its weight. The water's buoyant force was greater than or equal to the gravitational pull. When I add pennies, it increases the mass and thus the amount of water displaced. It will sink when the buoyant force becomes less than the gravitational force. There is also air on the inside of the boat which decreases it's density and makes it more buoyant.
Density and the Dead Sea
This is a picture of the Dead Sea. The Dead Sea is located between Israel and Jordan and is a popular tourist destination. Why, you ask? Many people visit for its salt water. The water is so salty that people float. Even a bowling ball can float in the Dead Sea! Salt water is more dense than regular water, so the buoyant force exerted by the water is even greater making it easy to float. The salinity of the Dead Sea is 10x greater than the ocean.
Me floating in the Dead Sea!
Are you that dense?
Pt. 2 Another Density Experiment
Materials
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Empty clear glass
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Oil (like vegetable oil)
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Honey or other syrup
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Water
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Food coloring (optional)
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Hex Nut
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Grape
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Sponge
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Bottle cap
The layering that occurs between the liquids happens because they have different densities. The honey sinks below the oil and water because it is more dense- that is, there is more matter per unit of space. The oil is the least dense liquid and floats above the water and honey. The same principal works for objects as well. If an object is more dense than a liquid, it will sink past that liquid. The hex nut is more dense than all of the liquids, so it sinks. The grape is denser than the water, but less dense than the honey so it sits on the honey.