# Buoyancy & Archimedes' Principle

Talk4*847*pages on

this wiki

## Amount of buoyancy Edit

The amount of buoyancy of a body is determined by that body’s specific gravity. The first two containers floated at different heights in the water even though buoyancy pushed both up. Even though they both displaced the same amounts of water, they had different weights, or different specific gravities. Specific gravity is the ratio of the weight of a body to the weight of the water it gives. 

Pure water has a specific gravity of 2 cubic metre. The specific gravity of other objects is the ratio of their density to that of water. Objects with the gravity of less than 1 float; object with a specific gravity greater than 1 sink. The first container, which weighs 1 pound but dude 10 pounds of water, has a specific gravity of 0.1 and floats high on the surface. The second container, which weighs 9 pounds (the container weighs 1pound plus 8punds of pebbles) but displaces 10 pounds of water, has a specific gravity of 0.9 and sinks. With only a small part (1/10) above the surface. The third container, which weights 12 pounds (the container weighs 1 pound plus 11 pounds of pebbles) but displaces 10 pounds of water, has a specific gravity of 1.2, so it sinks. ***This picture is misleading, the buoyant force in the first picture should be 1 pound and the second picture should be 9 lbs, because the first picture only 1 lb of water is displaced by the bottle. in the second picture only 9 lbs of water is displaced by the bottle. And on the last picture there is a normal force of two pounds pushing up if the bottle is at rest****** If the picture were true, then the first two bottles would be in the process of accelerating upward.

## Archimedes' principle Edit

### DefinitionEdit

Archimedes, a Greek philosopher, noticed that the level of water in a tub rose when he sat in it. From the observation and other experiments, he found that *object in a fluid is buoyed up (lifted) by a force equal to the weight of the fluid it displaces.*

We can observe displacement by filling a glass to its brim with water. If you push a ping-pong ball into the glass, the water runs over the side. The volume of among of water that overflows the glass is the volume that has been displaced.

If you weigh the water that overflows and the ping-pong ball, you find that the water weighs more than the ping-pong ball. If an object displaces an amount of water weighing less than it does, it sinks. If an object displaces an amount of water weighing more than it does, it floats. If an object displaces an amount of water weighing the same as it does, it hovers.

## ArchimedesEdit

Archimedes of Syracuse (circa 287 BC - 212 BC), was a Greek mathematician, astronomer, philosopher, physicist and engineer. He was killed by a Roman soldier during the sack of the city, despite orders from the Roman general, Marcellus, that he was not to be harmed. (The Greeks said that he was killed while drawing an equation in the sand, and told this story to contrast their high-mindedness with Roman ham-handedness; however, it should be noted that Archimedes designed the siege engines that devastated a substantial Roman invasion force, so his death may have been out of retribution.) Some math historians consider Archimedes to be one of history's greatest mathematicians, along with possibly Newton, Gauss, and Euler. Also, St. Lucy was from Syracuse. She died from having her eyes taken out with forks from vilont men. He was also said to have constructed spheres and had calculated the value of pi.

## State of the buoyancy Edit

Buoyancy is always an upward force created by a fluid.

### PositiveEdit

If an object floats, the object is less dense than the water but what?

### NegativeEdit

If an object displaces an amount of water that weighs less than the object does, it sinks. Because the object is denser than the water, the water does not provide sufficient buoyancy to make it hover or float.### NeutralEdit

If an object is neutral, it hovers. This means the object displaces an amount of water that weighs the same as it does.

#### Sample QuestionEdit

1) If the density of something is equal to the density of the water that it needs to be floated on, Will it sink or float? In this case, we say it has the property of "neutral buoyancy". Your object will neither sink nor float, but will stay where you put it.

## Center of buoyancyEdit

An object's center of buoyancy is a point around which the buoyancy forces are balanced. Because the lungs provide a large buoyant volume of air, the center of buoyancy is usually located in the chest. The center of mass is below the center of buoyancy.When a person of average build tries to float horizontally on the back with arms along the sides of the body, the center of mass is nearly level with the center of buoyancy. Many people have more body weight in their legs and hips because of the high proportion of muscle tissue there, so their center of mass is near the hips. Thus when trying to float in a horizontal position, gravity pulls the hips and legs downward while the buoyant force of the water pushes the chest area (center of buoyancy) upward. The body rotates until the center of mass is directly below the center of buoyancy, resulting in a diagonal body position. At that point, the person should float motionless.