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KEY POINTS
A body in motion stays in motion, moving at the same speed and direction, unless acted on by a force.
The energy of an object and its potential to do damage are directly proportional to its mass and the square of its velocity 
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The total energy of a system cannot be destroyed, although it may be transferred between objects or transformed into work.
All injuries can be explained by four different types of strain (tensile, shear, compressive, and/or overpressure).
If a force causes a decrease in the original volume of a fluid- or air-filled structure, there must be a concomitant increase in the pressure.
The shape (and thus the sectional density) of a bullet is subject to change within the tissue and directly impacts the degree of tissue damage.
A projectile creates both a permanent cavitation or “bullet track” and a hydrostatic pressure wave that stretches and damages adjacent tissues.
In explosions, more energy is transferred if the shock wave travels through a medium of similar density (eg, water and the human body).
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Although patients suffer what initially may appear to be a unique constellation of injuries, all trauma can be described using physical principles. As such, understanding how the physical properties of force, mass, and velocity contribute to energy transfer can help the treating surgeon anticipate, identify, and subsequently treat injuries based on mechanism. This chapter will discuss the basic laws of physics that dictate the interaction between the injuring mechanism and the victim. This is followed by a discussion of blunt trauma, with special consideration of specific body regions and populations. Finally, the science of ballistics and the injuries caused by firearms and explosions are presented.
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BASIC PRINCIPLES OF MOTION
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In the late 17th century, the English physicist Sir Isaac Newton (1642–1727) first described how mechanical events could be explained using laws of motion and gravity. This section will provide a brief overview of Newtonian physics and how its principles directly apply to injury.1
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Newton’s first law is as follows: Every object persists in its state of rest or in uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.
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According to Newton’s first law, a body in motion stays in motion, moving at the same speed and direction, unless acted on by a force. The state of being in motion, otherwise known as momentum (p), is described by both the object’s mass (m) and its velocity (v).
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A motorcycle crash provides a classic example of this principle. Both rider and motorcycle have their individual mass and are traveling at the same velocity. When the motorcycle encounters an object, its velocity becomes zero. In contrast, the rider continues ...