What is the Equation of Density and Why Objects Float in Water
By: Michael R.
September 28, 2012



History

In 250 BC, a Greek mathematician Archimedes was given the responsibility of figuring out whether the Craftsmen had defrauded the king. It was said that the Craftsmen had put some silver in with the crown. When Archimedes was getting in a bathing pool he noticed that the water was overflowing when he was in the pool. He had an epiphany. The water that was overflowing was how much space his body occupied. He placed the king’s crown in water and a solid gold crown in another. And the gold crown overflowed less so the king’s crown was made with some silver. Silver occupies more space.

Density

Density is the amount of matter contained in a certain volume. The equation of density is density=mass and volume. Every object has density anything that is less than the density of a liquid can float and anything that has a higher density than the liquid will sink. When the water is hot or cold it has different densities. Solids are denser than liquids and liquids are denser than gasses. Oil can float because it is less dense than water. The Earth is five times denser than water.


Upthrust

The water pushes up so the object floats. This force is called upthrust. Upthrust is what keeps an object a float. Upthrust is the water that is pushing back up against the object. Upthrust is what can help make an object float. The amount of upthrust depends on the amount of water the object displaces. Upthrust is greater on an object that is heavy. When an object sinks lower the up thrust increases. When an object is hollow it can sink deeper in the water.

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Figure 1: Upthrust


Ships

Ships float because of the shape and that it is hollow on the inside of it even though it is made out of steel. The shape of the hull and the front make it so that the weight is put in different places so that the upthrust is greater. The bow is almost at a point to make it easier to move, to help it stabilize, and to keep it up. Since it is hollow it can sink deeper in the water than if it were solid. Big ships don’t toss and turn in the waves because it has a compartment that fills up with water. Ships don’t sink easily because of the amount of upthrust.
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Figure 2: Diagram of a Ships Hull

For More information here are some sights:
Equation for Density and Why Things Sink or Float
Equation for Density and Why Things Sink or Float Part 2


Glossary

Density: the state or quality of something, compactness

Epiphany: a moment where you have figured something out or an exciting moment

Hull: the bottom of a boat or ship

Upthrust: a thrust in an upward direction


Citations

Blachford Stacey. The Gale Encyclopedia of Science vol. 2. Detroit: Gale Group, 2001. Print.

"Density." Compton's by Britannica. Encyclopædia Britannica Online School Edition. Encyclopædia Britannica, Inc., 2012. Web. 13 Sept. 2012.

Kerrod Robin, Sharon Anne Holgate. The Way Science Works. New York, NY: DK Publishing, 2002. Print.

Macaulay David. The Way Things Work. Boston: Houghton Mifflin, 1988. Print.

Marie Day Martha. Density. Vision Learning. National Science Foundation, 2002. Web. September 26, 2012.

McGrath Kimberley A. The World of Physics. Detroit: Gale, 2001. Print.

National Geographic Society. The nature of matter. D.C.: Education Division, 2002. Print.

"Water." Compton's by Britannica. Encyclopædia Britannica Online School Edition. Encyclopædia Britannica, Inc., 2012. Web. 13 Sept. 2012.