The History of Forensic Physics
The word “forensic” comes from the Latin adjective “forensis” meaning of or before the forum. During the time of the Romans, a criminal charge meant presenting the case before a group of public individuals in the forum. Both the person accused of the crime and the accuser would give speeches based on their side of the story. The individual with the best argument and delivery would determine the outcome of the case. Basically, the person with the sharpest forensic skills would win. This origin is the source of the two modern usages of the word "forensic" - as a form of legal evidence and as a category of public presentation.
The application of physics for purposes of civil or criminal law. Forensic science has been dominated by the fields of chemistry (mainly analytical chemistry) and biology (serology and DNA profiling). Indirectly, physics has contributed to forensic science via the invention of the microscope, the electron microscope, the mass spectrometer, and optical spectrometers; but directly, its role in forensic science has been minimal. Forensic physics has traditionally involved the measurement of density (soil and glass examination), index of refraction, ballistics, and birefringence (fiber analysis, glass examination).
The ballistic pendulum was developed about 1743 by Robins, who was the first to undertake a systematic series of experiments to determine the velocity of projectiles. The principle of the ballistic pendulum, as well as of the gun pendulum, which was developed by Thompson, is the transfer of momentum from a projectile with a small mass and a high velocity to a large mass with a resultant low velocity.
To determine the velocity of weapons, the ballistic pendulum was invented. It consisted of a massive plate of iron to which was bolted a block of wood to receive the impact of the projectile; the pendulum was suspended freely from a horizontal axis. The block, when struck by the projectile, recoiled through a certain arc that was easily measured. Knowing the arc of recoil and the masses of the projectile and the pendulum, the velocity of the projectile could be determined by calculation. The ballistic pendulum was able to withstand the impact of musket balls only; however, by determining the relations that should exist between the caliber, length of barrel, and charge of power, Robins substantially advanced the science of gunnery. Later, the velocity of a projectile was determined by measuring the time required for it to travel a known length of its path. Numerous machines have been devised for this purpose; in 1840 the British physicist Sir Charles Wheatstone suggested the use of electricity for measuring small intervals of time. This suggestion led to the development of the chronograph, a device for recording, by electrical means, the time required for a projectile to pass between two screens of fine wire.
The development of high-speed photography and of the stroboscope by the American engineer Harold Eugene Edgerton and others has led to greater understanding in all three branches of ballistics. By means of such devices any projectile can be photographed in flight, thus permitting accurate studies not only of its velocity but also of its position and even of the shock waves it produces.
The most important recent development in ballistics is the use of computers. The calculus of exterior ballistics generally involves sets of second-order partial differential equations. Solving such a set of equations typically involves hundreds of thousands of computations. To find the position of the projectile at various points along the trajectory, dozens of such solutions are required. For each of various elevations of the gun, the entire process must be repeated. Even with the aid of slide rules and ordinary calculating machines, such an operation would take a mathematician an inordinate amount of time. Electronic computers compile complete solutions within a few seconds. Computers are used also for simulation of missile flights.
The design, development, and calibration of a wide variety of highly specialized optical and electronic equipment in recent years have furthered considerably the advance of all ballistics research, particularly with respect to the performance of guided missiles. Examples of these instruments are long-focus tracking telescopes, photogrammetric cameras, and miniature radio transmitters and receivers installed in missiles.