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Kin·e·mat·ics (kn-mtks) n: The branch of mechanics that deals with pure motion without reference to the masses or forces involved in it. From Greek knma, knmat-, movement.1

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As can be presumed from the derivation of the word kinematics, its essence revolves around motion. All injury is related to the interaction of the host and a moving object. That object may be commonplace and tangible, such as a moving vehicle or speeding bullet or more subtle as in the case of the moving particles and molecules involved in injury from heat, blasts, and ionizing radiation. Newtonian mechanics, the basic laws of physics, and the anatomic and material properties of the human body explain many of the injuries and injury patterns seen in blunt and penetrating trauma. Injury is related to the energy of the injuring element and the interaction between that element and the victim. Although most patients suffer a unique constellation of injuries with each incident, there are quite definable and understandable energy transfer patterns that result in certain predictable and specific injuries. Knowing the details of a traumatic event may aid the treating physician to further investigative efforts to uncover occult but predictable injuries.

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This chapter has been organized in a stepwise fashion. First, the basic laws of physics and materials that dictate the interaction between the victim and the injuring element are reviewed. This is followed by a more detailed examination of penetrating and blunt trauma and a synopsis of mechanisms specific to organs and body regions. It is hoped that this will offer the reader a better understanding of specific injury patterns, how they occur, and which injuries may result.

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Newton’s Laws, Impulse, Momentum, Energy and Work, Elastic and Inelastic Collisions

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Newton’s first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is the definition of inertia. Newton’s second law builds on the first and further defines a force (F) to be equal to the product of the mass (m) and acceleration (a).

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The application of a force does not occur instantaneously, but over time. If we multiply both sides of the above equation by time

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The product of force and time is known as impulse and multiplying acceleration by time yields velocity. Momentum (p) is defined to be the mass (m) of an object times its velocity (v).

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hence

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The important fact is that a force or impulse will cause a change in momentum and, likewise, a change in momentum will generate a force.2 This folds into Newton’s third law, which states that for every action or force there is an ...

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