For the viewpoint of physics (dynamics, to be exact), a firearm, as are most weapons, is a system for delivering maximum destructive energy to the target with minimum delivery of energy and momentum back to the shooter. Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. ... In physics, dynamics is the branch of classical mechanics that is concerned with the effects of forces on the motion of objects. ... Some firearms A firearm is a kinetic energy mechanical device that fires either single or multiple projectiles propelled at high velocity by the gases produced by action of the rapid confined burning of a propellant. ... The bayonet, still used in war as both knife and spearpoint. ... In physics, momentum is a physical quantity related to the velocity and mass of an object. ...

It should be noted that the impact to the target can be no greater than the impact of the recoil, due to the law of conservation of momentum. However, the smaller size of the bullet, compared to the gun-and-shooter system, allows significantly higher energy to be imparted to the bullet than to the shooter, giving guns their lethal effect. Consider a system where the shooter-and-gun have mass M and the bullet has mass m, and the two systems move away from one another with new velocities V and v. The recoil when firing a gun is the backward momentum of a gun, which is equal to the forward momentum of the bullet or shell, due to conservation of momentum. ... In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. ...

Then, by the law of conservation of momentum, MV = mv, and thus V = mv/M and the kinetic energies imparted to the two systems are respectively 1/2 MV² and 1/2 mv². The energy imparted to the shooter can then be written as This article may be too technical for most readers to understand. ...

Thus the ratio of the energies is the same as the ratio of the masses of the bullet and the shooter. However, almost all of the energy will be dissipated in the target.

When the bullet strikes, its high velocity and small area means that it will exert large stresses in any object it hits. This usually results in its penetrating any soft object, such as flesh. The energy is then dissipated in the wound tract formed by the passage of the bullet. See terminal ballistics for a fuller discussion of these effects. Stress tensor In physics, stress is the internal distribution of forces within a body that balance and react to the loads applied to it. ... Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a kinetic energy projectile when it hits its target. ...

The reversal of this mathematics is the reason why bulletproof vests work: the vest's material, usually Kevlar, works by presenting a series of material layers which catch the bullet and spread its momentum, hopefully bringing the round to a stop before it can penetrate into the body. A bullet resistant vest – also called body armour (U.S. body armor) – is an article of protective clothing that works as a form of armour to minimize injury from being hit by a fired bullet. ... Kevlar, also known as Twaron and poly-paraphenylene terephthalamide, is a synthetic fibre that is five times stronger than steel, weight for weight. ...

People who wear this kind of vest also have to remember that while it can prevent a bullet from penetrating, they can still be affected by the kinetic energy of the bullet which can produce serious internal injuries.

Hollywood depictions of firearm victims being thrown through plate-glass windows are inaccurate, as were this to be the case, the shooter would also be thrown backwards with equal force. Gunshot victims frequently fall or collapse when shot; however this is usually due to physical damage or psychological effects perhaps combined with being off-balance, not the momentum of the bullet pushing them over.

(Note that the above paragraph does not apply if the victim is hit by heavier projectiles such as 20 mm cannon shell, where the momentum effects can be enormous; this is why very few such weapons can be fired without a weapons mount.)

See ballistics for a more technical discussion. It has been suggested that Forensic ballistics be merged into this article or section. ...