Buoyancy and Ghosts of Mars: Difference between pages

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{{Infobox_Film |

In [[physics]], '''buoyancy''' is the upward [[force]] on an object produced by the surrounding [[fluid]] (i.e., a [[liquid]] or a [[gas]]) in which it is fully or partially immersed, due to the [[pressure]] difference of the fluid between the top and bottom of the object. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least to seem lighter. Buoyancy is important for many [[vehicle]]s such as [[boat]]s, [[ship]]s, [[balloon]]s, and [[airship]]s.

name = John Carpenter's Ghosts of Mars |

==Forces and equilibrium==

image =Ghostsofmars01.jpg |

imdb_id =0228333 |

writer =[[John Carpenter]]<br>Larry Sulkis |

starring =[[Ice Cube]]<br>[[Natasha Henstridge]]<br>[[Jason Statham]]<br>[[Pam Grier]]<br>[[Clea DuVall]]<br>[[Joanna Cassidy]]|

director =[[John Carpenter]] |

producer =Sandy King |

distributor =[[Storm King]] [[Screen Gems]] |

released =[[August 24]], [[2001]] ([[Theater]])<br>[[December 4]] [[2001]] ([[DVD]]) |

runtime =98 min. |

language =English |

music =[[John Carpenter]] |

awards = |

amg_id = 1:250566 |

budget =$28,000,000 |

}}

'''''Ghosts of Mars''''' (also known as '''''John Carpenter's Ghosts of Mars''''') is a [[2001]] movie directed by [[John Carpenter]], which in its basic themes is similar to his earlier [[Assault_on_Precinct_13_(1976_film) | Assault on Precinct 13]].

Buoyancy provides an upward force on the object(like floating). The magnitude of this force is equal to the [[weight]] of the displaced fluid. (''[[Displacement (fluid)|Displacement]]'' is the term used for the weight of the displaced fluid and, thus, is an equivalent term to buoyancy.) The buoyancy of an object depends, therefore, only upon two factors: the object's [[volume]], and the [[density]] of the surrounding fluid. The greater the object's volume and surrounding density of the fluid, the more buoyant force it will experience. If the buoyancy of an (unrestrained and unpowered) object exceeds its weight, it will tend to rise. An object whose weight exceeds its buoyancy will tend to sink.

==Plot==

The atmosphere's density depends upon altitude. As an [[airship]] rises in the atmosphere, therefore, its buoyancy reduces as the density of the surrounding air reduces. The density of water is essentially constant: as a [[submarine]] expels water from its buoyancy tanks (by pumping them full of air) it rises because its buoyancy stays the same (because the volume of water it displaces stays the same) while its weight is decreased.

The movie is set in the mid 22nd century. The planet [[Mars]] has been [[terraformed]], allowing Humans to walk on the surface without having to wear pressure suits. The story concerns a police officer, Melanie Ballard ([[Natasha Henstridge]]) who is leading a small team to pick up and transport a prisoner named Desolation Williams ([[Ice Cube]]). Arriving at the remote mining town where Williams is being held, Ballard finds virtually all of the people missing. On investigating she discovers that the miners found an underground doorway which had been created by an ancient [[Martian]] civilization. When the door was opened it released "ghosts", disembodied spirits who possessed the miners.

As a floating object rises or falls the forces external to it change and, as all objects are compressible to some extent or another, so will the object's volume. Buoyancy depends on volume and so an object's buoyancy reduces if it is compressed and increases if it expands.

Violence ensues, as the now possessed miners commit acts of [[death]] and [[destruction]], as well as [[self-mutilation]]. <!--The self mutilation however may be merely the alien spirits attempting to alter their human bodies to look more like those of the aliens themselves (which are only partially seen very briefly).-->

If an object's [[compressibility]] is less than that of the surrounding fluid, it is in stable equilibrium and will, indeed, remain at rest, but if its compressibility is greater, its equilibrium is [[unstable]], and it will rise and expand on the slightest upward perturbation, or fall and compress on the slightest downward perturbation.

Ballard must fight off the attacking miners, escape the town, and if possible destroy the ghosts. Unfortunately all these tasks are complicated by the fact that killing a possessed human merely releases the Martian spirit, which immediately possesses another human. Eventually they decide to blow up a [[nuclear]] reactor, which kills the human hosts. All of Ballard's team are killed by the miners, leaving only her and Desolation. Not wanting the authorities to blame the massacre on him, he handcuffs Ballard to her bed and escapes the train, leaving her to return home. While she reccuperates at a hospital, the miners attack the city. Desolation returns with a pair of [[nickle]] plated [[uzi]]s and they team up to fight off the alien zombies, setting the film up for an unlikly sequel.

A submarine is more compressible than the surrounding water.{{Fact|date=March 2007}} As depth increases, the resulting pressure causes the submarine's volume to decrease more than the volume of the surrounding water decreases. Buoyancy depends upon the object's volume and the weight of the displaced fluid. Volume has decreased so the weight displaced has decreased which means a decrease in buoyancy and the submarine tends to sink further. A rising submarine expands more than the surrounding water, the submarine tends to rise further.

The height of a balloon tends to be stable. As a balloon rises it will tend to increase in volume with reducing atmospheric pressure. But the balloon's cargo will not expand. The average density of the balloon decreases less, therefore, than that of the surrounding air. The balloon's buoyancy reduces because the weight of the displaced air is reduced. A rising balloon tends to stop rising. Similarly a sinking balloon tends to stop sinking.

The buoyant force can be expressed using the following equation:

==Production Notes==

:<math>F_\mathrm{buoyant} = - \rho V g \, </math>

*Although Mars has a day/night cycle almost identical in length to Earth's, most of the movie is set at night. The only moment in which we see Mars during the daytime is in a flashback through the character of the scientist, when she talks about how she found and opened a "Pandora's Box" that let the alien spirits out.

where

*Much of the movie was filmed in a [[gypsum]] mine in New Mexico. The pure white gypsum had to be dyed with thousands of gallons of biodegradable red food dye to recreate the appearance of the Martian landscape.

==Trivia==

: <math>\rho \, </math> is the density of the fluid;

*Originally Courtney Cox was set to play Ballard but after her foot was run over accidently by her husband she was replaced by Natasha Henstridge.

: ''V'' is the volume of the object submerged;

*On a similar note, Jason Statham was originally set to play Desolation Williams. This film also contains Statham's first on screen kiss.

: ''g'' is the [[standard gravity]] (<math>\approx</math> 9.81 [[newton|N]]/[[kilogram|kg]] on [[Earth]]). Negative sign must be used since the buoyancy is opposite in direction with the acceleration due to gravity.

==External links==

==Archimedes' principle==

[[image:Submerged-and-Displacing.png|right]]

Archimedes' principle, or the law of upthrust, is:

:''The rule that the weight of fluid that a submerged object displaces is the same amount of buoyancy force applied to the submerged object.''.

*{{imdb title|id=0228333|title=Ghosts of Mars}}

It is named after [[Archimedes]] of [[Syracuse, Italy|Syracuse]], who first discovered this law. [[Vitruvius]] ([[De architectura]] IX.9–12) recounts the famous story of Archimedes making this discovery while in the bath (for which see [[Eureka (word)|eureka]]) but the actual record of Archimedes' discoveries appears in his two-volume work, ''On Floating Bodies''. The ancient [[Chinese people|Chinese]] child prodigy [[Cao Chong]] also applied the principle of buoyancy in order to measure the accurate weight of an elephant, as described in the [[Sanguo Zhi]].

*[http://www.theofficialjohncarpenter.com/pages/themovies/gm/gm.html Ghosts of Mars at theofficialjohncarpenter.com]

{{John Carpenter Films}}

The weight of the displaced fluid is directly proportional to the volume of the displaced fluid (specifically if the surrounding fluid is of uniform density). Thus, among objects with equal masses, the one with greater volume has greater buoyancy.

Suppose a rock's weight is measured as 10 newtons when suspended by a string in a vacuum. Suppose that when the rock is lowered by the string into water, it displaces water of weight 3 newtons. The force it then exerts on the string from which it hangs will be 10 newtons minus the 3 newtons of buoyant force: 10&nbsp;&minus;&nbsp;3 = 7 newtons. This same principle even reduces the apparent weight of objects that have sunk completely to the sea floor, such as the sunken battleship [[USS Arizona (BB-39)|USS ''Arizona'']] at [[Pearl Harbor]], Hawaii.

The density of the immersed object relative to the density of the fluid is easily calculated without measuring any volumes:

:<math>\mbox{Relative density} = \frac { \mbox{Weight} } { \mbox{Weight} - \mbox{Apparent immersed weight} }</math>

==Density==

If the weight of an object is less than the weight of the fluid the object would displace if it were fully submerged, then the object has an average density less than the fluid and has a buoyancy greater than its weight. If the fluid has a surface, such as water in a lake or the sea, the object will float at a level so it displaces the same weight of fluid as the weight of the object. If the object is immersed in the fluid, such as a submerged submarine or a balloon in the air, it will tend to rise.

If the object has exactly the same '''density''' as the liquid, then its buoyancy equals its weight. It will tend neither to sink nor float.

An object with a higher average density than the fluid has less buoyancy than weight and it will sink.

A ship floats because although it is made of steel, which is more dense than water, it encloses a volume of air and the resulting shape has an average density less than that of the water.

==Applications==

* [[Anderton Boat Lift]]

* [[Falkirk Wheel]]

* [[Neutral Buoyancy Laboratory]]

==See also==

* [[Buoyancy compensator]]

* [[Cartesian diver]]

* [[Diving weighting system]]

* [[Hull (ship)]]

* [[Hydrometer]]

* [[Lighter than air]]

* [[Naval architecture]]

* [[Negative buoyancy]]

* [[Pontoon]]

* [[Quicksand]]

* [[Submarine]]

* [[Thrust]]

==External links==

{{wiktionary}}

*[http://www.bigs.de/en/shop/htm/wssab01.html Falling in Water (Animation 1)]

*[http://www.bigs.de/en/shop/htm/wasser01.html Falling in Water (Animation 2)]

*[http://www.newton.dep.anl.gov/askasci/phy99/phy99x88.htm Falling in Water]

{{2000s-horror-film-stub}}

[[Category:Fundamental physics concepts]]

[[Category:Diving]]

[[Category:Introductory physics]]

[[Category:Ship construction]]

[[Category:2001 films]]

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[[Category:Mars in fiction]]

[[Category:American films]]

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[[Category:Films directed by John Carpenter]]

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[[Category:Ghost films]]

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