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Accident Reconstruction Education Information Accident Reconstruction Dictionary Click on the letter below to find names and definitions of accident reconstruction and safety terms that begin with that letter.
A's Acceleration: a change in velocity with respect to time. Values are either positive or negative with standard orientation. Acceleration Scuff: friction marks that occur when the tires on the drive wheels of the vehicle are turning faster than they would normally turn, given the speed of the vehicle. Airlockers: air operated differential locks for traction purposes Angular Momentum: a quantity associated with how an object moves around a reference point. It is often used to describe rotating objects. The angular momentum of an object is defined to be equal to its mass times its velocity about the point times its distance from that point. Approach Angle: as a vehicle approaches a grade, it's the angle of slope which it can climb up without any part of the front (bumper, shackle etc.) hitting the ground/rock Articulation: the extent of movement between one tire and the other on the same axle Asphaltic Concrete: a flexible roadway surface that meets the needs of most highways. It is constructed of an asphalt and rock mixture and is black in appearance. Axis of Rotation: in a rotating object, the axis of rotation is the line about which the object is turning. The axis of rotation for a spinning record is its middle. The reference point for angular momentum is often chosen to be along the axis of rotation. B's Breakover Angle: when a vehicle crests a hill, it's the angle at which the front tires still remain on the ground as the rear wheels continue to climb (see High Centered) C's Chord: a straight line that intersects a circle at two (2) points. Coefficient of Kinetic Friction: ratio of the magnitude of the force of kinetic friction to the magnitude of the normal force. Collision Scrub: occurs during a traffic collision when the downward and rotating forces cause a smearing of tire material on the road surface. Conservation: the idea that the total amount of certain quantities in nature always stays the same. Energy, linear momentum, and angular momentum are examples of quantities that obey the law of conservation. Conserve: stay the same or constant. To say that something is conserved means is to say that the amount of it stays the same and obeys conservation. Coordinate Method: technique of scene documentation where items of evidence are located by measuring the distance from two (2) reference lines. Similar in concept to locating points in the Cartesian plane. D's Departure Angle: the angle between your rear tires and the first rear protrusion that would scrape the ground as you descend off of a hill Differential: also referred to as "pumpkin" it's the device that transmits power to two points allowing one to go faster than the other Divided Attention: the ability or lack thereof to do more than one thing at the same time. E's Energy: a quantity associated with a system. Energy is often related to the motion of an object or the potential for motion. Kinetic energy, potential energy, chemical energy, and heat energy are examples of forms of energy. F's Floating Axle: in the "full" case, the drive shaft is independent (floating) of the hub bearings, in the "semi" state the shaft supports the hub bearings Force: an influence upon an object that causes motion or a change in motion Four Wheel Drive: all four wheels are driven (also referred to as 4wd or 4x4 4by4) Four Wheeling Hub: hubs are devices in the front wheels which makes you able to disengage the front drive train, to free wheel, and when needed, lock the hubs in for four wheeling situations Friction: the resistance of an object to movement across a surface. A way that a form of energy, such as kinetic energy, is changed into another form, usually heat energy. Friction causes you to slow down when you are swinging. It also causes a spinning top to eventually fall over. G's Gravitational Acceleration (g): the constant describing the acceleration of any object falling toward the earth. Near the earth, gravitational acceleration is approximately equal to 9.8 meters per second per second (m/s2). Gravitational Potential Energy: a type of potential energy. Gravitational potential energy is associated in the interaction of an object with the earth. It is defined to be equal to the mass of the object times the gravitational acceleration times the distance of the object from the ground. Ground Clearance: this is the amount of space between the lower most hanging part of your vehicle's undercarriage and the flat ground H's High Centered: when obstacles or terrain are broached by the front tires and the vehicle is suspended on it's undercarriage so that three or more tires lack traction I's Impending Skidmarks: also known as shadow skidmarks are small fragments of rubber, ground or scraped from the tire, at the tire and roadway interface, as the rotating tire slows to a locked position. J's K's Kinematics: the branch of mechanics that uses mathematics to describe motion, without reference to the forces or masses involved. Kinetic Energy: a type of energy describing an object's motion. Kinetic energy is defined to be equal to 1/2 times the object's mass times the square of the velocity of the object. Kinetic Friction: describes the frictional forces present between surfaces in relative motion. L's Live Axle: an axle supporting the weight of the vehicle containing the power to the wheels Locker: or differential locker, is a device that forces the diff to allocate equal power to both points (both tires) thus keeping power to a slipping tire M's Middle Ordinate: the distance of the center point of the chord, to the outer edge of the inertial scuffmark. Used in determining the radius of the circle that most closely approximates the shape of the arc. Momentum: the mass of an object multiplied by its velocity. It is a vectored quantity with both magnitude and direction. N's P's Point of No Escape (by braking): consists of te perception/reaction distance plus te slide-to-stop distance for the specified velocity. Portland Cement Concrete: a rigid roadway surface designed for roadways that are continuously exposed to large trucks, large volumes of traffic and sections of roadway where the grade is 12 percent or greater. The color is normally white or grey. Potential Energy: a type of energy having to do with how two or more objects are interacting. Potential energy is stored in the interaction of an object with another object. Gravitational potential energy is a type of potential energy. Q's R's Raised Pavement Markers (R.P.M.): Non-reflectorized ceramic pavement markers, also known as "Bott's Dotts", are most often used to delineate travel lane lines. Reflectorized RPM's come in many different colors and are used to delineate travel lane lines and outside boundaries. Reference Point: in angular momentum, the point in space around which the motion of an object is described. The angular momentum of an object is defined in relation to some point, the reference point. The velocity and radius of an object are measured from the reference point. The reference point is often chosen to be along the axis of rotation of a rotating object. S's Scientific Method: 1.) make observations of available facts; 2.) develop a hypothesis; and 3.) test the hypothesis Scuffmarks: friction marks left on the roadway by tires that are free to rotate. Shackle: for recovery, it's the "U" shaped device with a screwable pin across the open top that results in a "D" shape so that the two ends of a strap can be securely attached from which the hook of winch cable can be attached as an anchor point Shackle (suspension): a coupling device usually found on leaf springs, used to attach the frame and the suspension component Short/Long Wheel Base: the manufacturers wheel base dimension measured from the center point of a front wheel to the center point of the same side rear wheel (wheelbase from front axle to rear axle) 'short' refers to "Wrangler" type lengths, long refers to "Grand Cherokee" type measurements Skidmarks: dark deposits of tire and roadway material normally deposited on a roadway surface by a tire that is not free to rotate. Snatch Strap: another name for the recover strap, in this case referring to a maneuver that carefully makes a concerted 'tug' between one vehicle and the stuck vehicle, to 'snatch' or 'yank' the vehicle forward or backward, over an obstacle or out of gripping terrain Speed: magnitude void of direction. How fast an object is moving Sporadic Trickle: the fluid trail that usually starts after impact aspersion and continues to the vehicles final rest position or until the fluid container is empty. Static Friction: describes the frictional forces present that are between surfaces at rest with each other. Strap: a specially designed band of material that can be connected between vehicles to pull one vehicle forward, there are different widths and strengths for different weight applications Suspension Travel: how far between the highest and lowest distance a wheel can 'travel' from full compression to full hang or droop System: a group of objects T's Transfer Case: the secondary gear box splitting the power to the front and rear of the vehicle Transmission: the unit transmitting power from the engine to the wheels Tree Strap: a small band of strap that is used around the base of a strong tree, a shackle point attached at the two ends, and then a winch cable can be connected for self recovery (this protects the tree - never put metal cable around a tree!) Triangulation: technique for scene documentation where items of evidence are located by measuring the distance between two (2) reference points. V's Velocity: a change in position with respect to time. Has properties of both magnitude and direction. The speed of an object in a certain direction. Velocity is usually measured in meters per second (m/s) Yaw: a sidewise movement of a vehicle in turning; movement of a vehicle in another direction than that in which it is headed; sidewise motion produced when centrifugal force exceeds traction force. Often the result of overreaction or exceeding the critical speed. Sometimes revealed by tiremarks on the roadway. Yawmark: a scuffmark made while a vehicle is yawing; the mark made on the road by a rotating tire which is slipping in a direction parallel to the axle of the wheel.
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