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TPE Police scientifique (1)

Cours gratuits > Forum > Forum anglais: Questions sur l'anglais || En bas

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TPE Police scientifique
Message de mattheboss posté le 06-03-2011 à 18:13:54
Bonjour à tous,
nous sommes 3 élèves de 1ère S et nous devons rendre nos TPE demain , une de nos deux matières est bien sûr l'Anglais.
Si vous pouviez corriger un ou deux paragraphes chacun ce serait vraiment sympa parce que nous ne pouvons pas vous demander de tout corriger.
Merci d'avance à ceux qui nous aideront !!!


The scientific police combines all the police's activities connected to the research and identification of authors and victims (and sometimes witnesses) of infractions and crimes, thanks to technical and scientific ways. All these scientific methods are they really reliable ? Are they self-important to incrimine a suspect ? We are going to study some of these methods, which are the Ballistics, science who deals with all missile's characteristics (trajectory, impact, nature...), the fingerprints, lines left by the end of our finger's lines, and analysis of tracks of blood with its revelation.


The terminal ballistics studies what happens when the missile reaches its target and the biological and physiological reactions which it pulls. For these studies, we use tonics (a material which possesses the same properties as the human muscular tissues). Among these tonics we can raise the use of the gelatin, the plastisine, board of fir tree, the skin of fir tree. But the most reliable remains the " ballistic soap ", its density is practically equal has that some human muscular tissue (density=1.6gcm3). But the scientific headways, these tonics have one an inconvenience to be a homogeneous middle whereas the human body is a heterogeneous and complex middle.
We speak about screen effect when the missile passes through an intermediate objective before reaching the final target. This phenomenon can be at the origin of a change of the profile of the wound, in particular when the missile splits up or deviates from its original trajectory by destabilizing.
To strengthen the reliability of their methods, the ballistics experts study bounces and tired outstrip of shooting servants of physical formules, but we shall not bring in detail in court of our personnal working.
Finally we can see that the terminal ballistics dresses so a lot of importance for the policemen and the servicemen, by allowing new concepts of weapons and ammunitions. According to the particular custom of the cartridge, the manufacturers have to find a good compromise between the penetration and the deformation of the missile to decrease at most the rebounds and optimize the vulnérant power.

The laboratory in ballistics contains 2 main tools:
- The macroscope of comparison, which serves to compare missiles and cartridges found on the scene of the crime to those fired in laboratory. The macroscope of comparison possesses two objectives, what allows to observe two objects at the same time. It makes slide the images the one on the other one: if there is juxtaposition between the images of 2 bullets, the expert concludes that the same weapon fired 2 missiles.
- The Ibis system, which intervenes after the comparison, allows to scan missiles and cartridges found, then we register these images in a database, the base C.I.B.L.E (Comparison and Ballistic Identification by Localization of Imprints). She allows to compare the marks(brands) of a suspicious missile with those the other criminal affairs. The experts can thus connect crime between them.

Modifié par lucile83 le 06-03-2011 18:14

Modifié par lucile83 le 06-03-2011 20:03
Je n'avais pas vu que c'était aussi long, et en fait, très bizarrement traduit, comme dit gerondif.Ce sont donc 2 raisons pour fermer ce topic,désolée.

Réponse: TPE Police scientifique de mattheboss, postée le 06-03-2011 à 18:14:47

A fingerprint is a mark left by the crests of fingers, hands, toes or feet when they touch an object.
Fingerprints begin to be formed from the seventh week of pregnancy, and are definitively drawn in the sixth month by the life in the womb They are unchanging, of a big variety. Indeed, they take no morphological modification or during the growth and disappear that by decomposition. Furthermore, that we burn, cuts itself, that we have bulbs or the others, the papillaires details are ceaselessly reconstituted in the identical. Unlike other indications, a fingerprint is unique. She cannot be stolen, borrowed or forgotten. If it is there, it is because the person to whom she belongs was present. Every imprint presents a large number of characteristics which are classified in types and subcategories: the main motives are Arches, Loop and whorl.

When a fingerprint is on a crime scene, we must determinate the fingerprint's nature:
-Latent (invisible to the eye): They are the one we usually find, it's a depositing (from sweat-gland or sebaceous-gland). To make they appear, you can use a chemical relevation (ninhydrin) or maybe a physical revelation (powders...)
-Shaped: When the finger touches a malleable subtance, as painting or others, the fingerprint is shaped. It's usually difficult to study, special method of lights are used to photography it.

Then we must determinate what type of fingerprint it is.
The most known are : Arch (or tented arch), Loop and Whorl
Those 3 types of fingerprints are about 95% of human's fingers, other types of fingerprints exist but they're less known.
The study of fingerprints is called the fingerprinting methods.
There is a file called FAED (File automated of fingerprints) which treat the image and compares it to those already present on the database, if the imprint appears on this file. It allows to identify approximately 130 individuals / month.
The concordance's points between fingerprints are called the accuracies. If two fingerprints are identical, we must be able to find a lot of accuracies
Connected to the experiment and the training, expertise's value would base on expert's value. The decision-making would be established on subjectives datas, it questions the reliability of this domain.

In France, « The formel identification » is pronounced when 12 concordance's points are found. This standard in 12 points takes its rules on a collection entitled « La preuve par les empreintes digitales » written by Edmond Locard who was released in 1914 where he says:
« Three orders of case can appear:
- 1) There is more than 12 points, the fingerprint is net, certainly for all the proof is useful in justice
- 2) There is between 8 and 12 points, borderline cases, cartainty is a function of: a) of the neatness of the fingerprint ; b) his type's rarity ; c) of the presence of figure's center or of the triangle in the decipherable part's ; d) of the pores' presence ; e)of the perfect identy of width of crests andfurrows, of the direction of the lines and angular value forks. In that case, the certainty leads only after discussion of the case by one or several completent and experimented specialists

- 3) There are few points: In that case, the fingerprint supplies no more certainty, but only an assumption, proportional to the points' number and their neatness. Edmond Locard: « The proof is by the fingerprints »
In 2007, a judge of the Maryland rected the proof by the fingerprints considering « The identification's process is not verifiable, not made out and subjective for being able to be claimed infallible ». This decision wasn't followed by the superiors authority, but the doubt settled down a little more still.
I 2009, the american academy of sciences returned his report: the « NAS report: Strenghtening forensic science in United States: A path forward », where it disadvantages some pratices of the scientific police in the United states, in particular in the fingerprints' field.
Among the multiple discussions, the cases of false identifications are discussed, because these cases exist, at least in the foreign countries.

We will describe one of the most recent and most known: the one of Brandon Mayfield:
The 11th March 2004, Spain is saddened by the death of 191 persons during a bomb attack in Madrid, During the investigations led by the scientific police's services, a fingerprint is revealed on a plastic bag which had contained detonators and found in a car. Unidentified by the spain's police, the track is sent to Interpol, then identified by the FBI as being the left index of someone called « Brandon Mayfield ». 15 concordance's points are found between the track on the plastic bag and And Brandon's left index's fingerprint. Arrested the 6th of May 2004, the suspect cries out his innocence, and was finally released two weeks two weeks later when the real person was identified by the spain's services

It's interesting because it's not one or two who commit an error but 3 experts from th FBI (and a fourth one but independant). They were all experimented in this domain. In front of this mistake, an investigation led by th « Service Inspector General » This last allows to list the causes of this false indentification. One of the main educations is the one of the influence bound to the way, that is to the impact which the incidental elements can have on the way the expert in fingerprinting methods will estimate the track.
However infallible the fingerprinting method can appear to us, we count cases of false identifications, what proves the limits of the fingerprinting methods in term of reliability. There is nothing shocking to approach the dactyloscopique proof with statistics and probability. Every day the decisions of justice are partially returned on the basis of statistics for another type of imprint: the genetic imprint. In view of the scientific rigor applied to the proof by the DNA, the proof by fingerprints is questioned today. THE IAI (International Association for Identification) go to the notion of probability having condemned her 30-year-old earlier. This statistical approach will only be this domain strengthening, offering him the degree of reliability, exactness for which a court is entitled to expect from a scientific proof.

Réponse: TPE Police scientifique de mattheboss, postée le 06-03-2011 à 18:15:21

I - Introduction
The blood is a red liquid, which circulates in all the bodies to bring the oxygen and the necessary and vital nutriments. It contains Iron and many other elements... We can note 4 blood groups: A, B, O, AB and two rhesus: positive and negative. On a crime scene, we often note the presence of tracks of blood, blood which arises from projections, from a flow since a wound or from a contact with a bloody object. By analyzing tracks and by reconstituating the way they arrived at this place, we can think about one or several possible scenarios and reconstituate the progress of the facts.

We can note three types of tracks of blood: passive tracks, thrown tracks, and transferred tracks (of contact).
Passive blood tracks : Passive blood tracks are the ones which are due to the gravity's force.
- Stain with dripping: spot creates by drops of blood falling in some blood.
- Stain with flow: spot creates when the shape and the direction of a bloodstain change because of the gravity or of the movement of the object.
- Pool of blood: spot creates when the source of blood remains immovable certain time
Thrown blood tracks: A thrown track occurs when a certain quantity of energy is transferred to a source of blood. We are going to speak about the "velocity" of impact of the tracks of blood. The velocity is a speed (in m/s) in connection with a direction. Here are the main types of projections of blood:
- Projection with low velocity ( PBV): the projections with low velocity are generally produced when an object moving unless 1,5 m/s returns in touch with a source of blood. The diameter of spots is generally upper to 3 mm.
- Projection with average velocity ( PVM): the projections with average velocity are generally produced when an object moving between 1,5 m/s and 7,5 m/s returns in touch with a source of blood. The diameter is generally situated between 1 mm and 3 mm. The objects which can produce this type of tracks are for example a knife or everything contendants objects.
- Projection with raised velocity ( PVE): the projections with high velocity are generally produced when an object moving in more than 30 m/s returns in touch with a source of blood. The diameter of the tasks is generally lower than 1 mm. These spots often give the impression to have been made by means of an airbrush. The projections with high velocity can be created by shots or explosives, but also by industrial machineries, cough, or sneeze.
- Projection due to the non-cooperation: arising spot when some blood is thrown by a person bleeding in movement.
- Stain with arterial spurting: arising spot when some blood escapes from the body by a split artery.
Transferred blood tracks (of contact) A transferred blood track (or of contact) occurs when an object full of blood contacts an object or a surface on which there is no blood. it isbe possible to discern the object which left the track.
- Stain with wiping: spot creates when an object returns in touch with an already present spot and which distorts its shape.
- Spot in the air: some blood is transferred by a source in movement towards a surface on which there is not.

III - Make the invisible visible

Even if the crime scene has been washed, we are able to vaporize a chemical revelation in the aim of making the luminescent zones appear where the blood has been cancelled. The one which we're going to use is called « luminol ». It reacts with hemoglobin without destroying DNA. Types of identified tracks, their localization may permiss us to establish a chronology of the murder.

The molecule called luminol is able to produce light when it's with hydrogen peroxide, which produce energy in the form of photons. This reaction is very slow, it can last a few months, it's when Iron which is contains in the blood is useful, it reacts as a catalist, so as the reaction is very fast. So Even if the luminol reacts with the hydrogen peroxide, iron is a must to make the reaction works good.

This reaction is an oxidation-reduction reaction, it works with an oxidizer (which raise electrons) and a reducer (which will give up electrons). In this reaction, the oxidizer is the hydrogen peroxide (H2O2) and the reducer is the luminol (C8H7N3O2) The acid-base pair is that: C8H7N302 / C8H5NO42- for the luminol, H2O2 / H20 for the hydrogen peroxide. The equation is the following one : 2HO- + 2H2O2 + C8H7N3O2 => 4(H20) + C8H5NO42- + N2

The products are water, ion 3-aminophtalate and diazote. The interesting one the the 3-aminophtalate ion (C8H5NO42-) which is in an excited state during this oxidation-reduction reaction. Being in an excited state, he will emit light particules: photons.

IV - Conclusion

It is impossible to erase the tracks of blood, even diluated 10000 times, the blood will always be visible by some ways in particular thanks to the improvement of Bluestar Forensic, The scientists know how to make the blood's tracks speak, they bring important information on the progress of a crime. The study of the blood is a reliable method in 100 % which could again strengthen itself thanks to the morpho-analysis of the tracks of blood which would be recognized as a scientific method in France soon.

Réponse: TPE Police scientifique de mattheboss, postée le 06-03-2011 à 18:15:40

Methods of scientific police, that is the ballistics, the fingerprints and the analysis of the blood tracks, are more and more successful in time. Thanks to the technological headways and to the new methods, the experts put less and less time to analyze the proofs and they are more safe. The ballistic stripes, but also the bloodstains are 100% reliable. It's impossible to cheat for these two methods. A fingerprint is unique, even twins have different imprints, it's impossible to remove an imprint of our fingers. The ballistic stripes are unique too, they are particular in every weapon, two bullets made in only some minutes of interval will be different because of the manufacturing defects. It's impossible to reproduce a bullet or to remove stripes, because the bullet wouldn't fire any more straight. We can't either remove bloodstains. Now, the experts have new techniques which still stress the reliability, such as the revelation with the luminol. We can deduct that the techniques of scientific police are fully tried and tested and are very reliable.

The policemen are more and more supported thanks to the technology, in particular thanks to fingerprints and ballistics databases who allow to compare the clues of a crime scene with those presents in databases. However the external parameters can't be taken into account, as scars and cuts for fingerprints. It's for that that the man always has to play a role and use his knowledge. It is the case in fingerprints analysis, where the study of fingerprints is left with the free will of one or several experts. However an error is possible, it's possible that an expert makes a mistake, we count some cases of errors of judgement in connexion with the experts mistakes. It's for this reason that the comparisons are made in the hand, and that there isn't a single expert but several to verify the proofs. We can't say that there is no risk of error, but the scientific technique is on the cutting edge of technology today because the scientific police methods were verified and proven. However, there are imperfections in particular in the field of the fingerprinting methods where one or several experts can make an error.

Réponse: TPE Police scientifique de gerondif, postée le 06-03-2011 à 19:38:06

comment avez-vous fait ce travail ? Il me semble que vous avez un peu abusé du traducteur parfois: par exemple:
Nous voyons que le terme "ballistique" revêt ainsi une grande importance pour les policiers.....
Finally we can see that the terminal ballistics dresses so a lot of importance for the policemen. Ca n'a pas de sens.

Beaucoup de mots semblent ainsi traduits et n'ont pas de sens:
"tired outstrip of shooting servants " (les distances de tir des valets de tir? (un valet de tir étant une sorte d'étau dans lequel on place l'arme pour que le tir soit toujours le même)) mais tired signifie "fatigué"....

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