The names and sizes of the measurement units used in each system are mainly what make them different. These are some of the units used for different measurements in each system.

	Now we can begin.  The metric system is based on a unit of 10, whereas the imperial system if given us or adopted from the
	the British system which is calso called the Imerial System.  We will see a conversion chart below:

So we now can calculete using conversion chart for the following: 

(Actual High School Physics question)

 				The Eiffle Tower in Paris is 986 feet tall. How tall is it in centirmeters?

	1 	inch = 2.24 cm;  1 Foot = 12 INCHES;  100 cm = 1 METER; 1,000M = 1K
		
	Therefore, 986 Ft = 986  x 12 = 11832 inches tall.  Now we have to convert inches into centirmeters.
	So, 11832 / 2.54 = 4658 cm
	Next to convert 4658 cm to Meters.  4658cm / 100 = 46.58m
	Finally, convert to Km.  Sot the Eiffle Tower standing 46.58 meters tall is 46.58 /1000Km = .046Km tall
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	Lets tak about the properties of Newtonian Physics.  These are the principles that an accident reconstructionist uses
in dedermining speed.  Tjere are several ways to determine motor vehicle speed.  Most common are skid, acceleration, yaw
marks which are left on the road surface due to skidding, acceleration, or centrifical (sliding tires while still rotating).

	Lets start with examles of skid marks, so we can determine how these marks wereleft on the road surface.

HALL - CARPENTER, CO.

By Edwin T. Scallon, Copyright © 1990, 1995, 2008 All Rights Reserved

ACCIDENT RECONSTRUCTION AND BAC CALCULATION PROGRAMS

IDENTIFICATION OF TIRE MARKS

       Tire marks, whether they are skid marks, acceleration marks, scrub marks, skip marks or curved yaw marks are rather simple to recognize once familiar with their patterns.  The following tire marks marks will indicate the direction of travel of the vehicle which left the marks and once measured will give either the minimum speed that the vehicle was traveL

SKID MARKS

  OVERLAPPING SKID MARKS                SKIP SKID                         CENTRIFUGAL YAW MARKS       

           

 SEMI-TRAILER LOCKED MAXIS

 

 

   OFFSET OR JOG MARKS            INTENTIONAL PAINTING              NO BREAKS ON TRACTOR                     

             

         SCALLOPED SKID                                    ACCELERATION MARKS

                   

Characteristics

    Skid marks are divided into "acceleration marks" created on acceleration, if the engine provides more power than the tire can transmit; "braking marks," if the brakes "lock up" and cause the tire to slide; or "yaw marks", if the tire slides sideways. Each skid mark has a characteristic appearance, and an experienced accident reconstructionist or forensic engineer can often determine what the vehicle was doing by examining the marks left by the tire.

    Contrary to popular belief, skid marks are not caused by pieces of rubber from the tire being deposited on the road because friction heats up the tire. Skid marks are actually bituminous oils in the asphalt that are heated because of the friction of breaking or accelerating and rise to the surface, leaving dark marks. This is why the marks fade. Within one hour, 3-4% of the mark has already faded.

    Braking marks will vary depending on whether the car uses ABS or not. If not, there will be two lines. The darker marks on the outside are from the front wheels, while the back tires leave thinner marks. This is because the force exerted makes the wheels take on a conical shape. If the car uses ABS, the marks will be much fainter as less heat is produced by the release-stop mechanism of ABS. Scuff marks may be visible even with ABS.

    Do you see any difference between skid marks and acceleration marks?  Looking at the beginning of the tire mark, all skid marks start out light and get darker as the skid continues.  Acceleration marks start out dark and get lighter as the vehicle reaches its momentum.  This is important, since as one case I investigated had a pedestrian pinned against a building.  The dead body's torso was draped over the hood of the car which was still against the building.  The operator claimed he tried to avoid the collision by skidding, but could not stop in time.  The tire marks showed heavy friction marks at the beginning of the alleged "skid" and got lighter as the car reached the pedestrian and the building.  Hence, the operator did not attempt to stop by skidding, but rather he committed murder by intentionally accelerating his car directly at the pedestrian. 

    Another tell tale sign of a skid mark versus an acceleration mark is the ability to steer while accelerating. A skid mark will always remain in a straight line through the center of mass of the vehicle in the direction the vehicle was traveling when entering the skid.  Note the pictures of the acceleration marks, the vehicles began accelerating and created either a paint pattern by steering or attempting to avoid collision while accelerating.  You can see in the Rail Acceleration photo, the race car has its front wheels turned while the rear tires are spinning.  Had the driver not backed off the acceleration, this race car would have begun a centrifugal yaw which would have resulted in the right rear spinning to the outside and the race car would have ended up pointing in the wrong direction.  There is one anomaly in the skid patterns.  Look at the tractor-trailer skid marks where you can see the dual skid marks of the trailer skidding, however, it is also changing direction.  This occurs more times than not when the tractor trailer driver has the front breaks removed from the tractor so he can steer the load around an object or preventing the rig from "jack-knifing".  This is not only illegal, but dangerous since it changes the breaking efficiency of the Tractor.  This is the only time a skid mark shows directional changes.

    Occasionally, you might see a long double pair of skid marks, obviously from a tractor-trailer.  This occurs when the air break houses that connect the tractor compressor to the rear tires of the trailer looses compression.  The trailer is equipped with what is known as "Maxi" breaks.  When the trailer rear tires loose compression, for what ever reason, the maxis engage and attempt to stop the rig.  The driver does not hear this and only becomes aware when his forward momentum is stunted or someone draws the driver's attention to the problem.  Another indication of tractor-trailer loads are the resulting skid marks.  Looking at the "skip-kid" marks left by the trailer portion of the tractor-trailer combination, they are intermittent skid marks.  This indicates that the trailer was empty (not load on board) and the trailer began to bounce during the skid. 

    Yaw marks a curved tire marks, they are actually scrub marks.  The tires usually all four are still turning but at the same time sliding sideways.  This occurs more times than not on a "hair-pin" curve where the operator realizes he is going too fast to negotiate the curve.  Instinctively, the operator tries to avoid going off the road and turns the steering wheel while breaking.  This action causes the vehicle to enter a centrifugal yaw, where the rear of the vehicle slides to the outside of the skid. The yaw marks have the distinct "hash marks" within the skid, as seen in the above photo.

    Scalloped ksid marks occur when the skidding tire looses inflation and the rubber tire collapses.  The scallop marks demonstrated in the above photo that this vehicle had a "blow-out" and began sto skid to a stop.

    Finally to round out the above tire marks is the "Off-set" or "Jog".  Many times untrained investigators look at the debris field at an accident scene.  They believe the point of impact, (POI), is where the majority of the dislodged vehicle parts are scattered on the road surface.  This is not an accurate determination of the POI.  Looking at the photo of the off set or jog, you can see an abrupt change of direction of the skid.  This is where the skidding vehicle came in contact with another object, most probably another vehicle traveling at a 90º angle to the skidding vehicle.  At POI the skidding vehicle skids in the direction of the striking vehicle.  If the vehicle strikes a fixed object like a building, the resulting tire mark is a scrub of the front tire which is wide and if the vehicle strikes the fixed object at an angle from the center of mass it will rotate in the opposite direction of the impact or to the side with the least resistance to the impact.  For example, a vehicle strikes a utility pole with the left front bumper, which is lef to the center of mass the vehicle's right rear will swing out to the right.

   Also it should be noted that although your eye may interpret that a vehicle when striking a fixed or semi-stationary object "bounces" this is not what happens.  Vehicles DO NOT bounce.  The energy that is contained in the system is dissipated by rotation, vaulting or deformation (crush of the vehicle striking parts).

________

It takes a little more than being abe to identify the tire marks, ALTHOUGH THE MOST IMPORTANT facit of speed from those marks.

The first thing one must realise is that GRAVITY is a constant. Second, gravity works through the center of mass of an object. In the case of a motor vehicle, one can determind the center of mass by neasuring the center of the vehicle. Once that is accomplished, one needs to find the "Frictional Coefficient of Drag" (How slipry or Rough" the road surface is at the time the tire marks were made. There are three acceptabe means of determining how rough or smooth the surface is. First, you could skid a vehicle on the same surface in the same diretion of the vehicle that left the marks, Second the use of a "drag sled" (most common), and finally the breaking efficiency of the vehicle.

Let's use a drag sled. It is a half cut rubber tire filled with cement to cover the interrior of the tire (about 6-8") of cement. See the photo below:

One would secure a chain to the eyebolt you mounted in the sled. At the other end you would use (typically) a fishing scale and pull the drag sled on the same surface the skid or acceleration marks were left in the same direction, up hill or down hill it makes no difference so long as you pull it on the same surface in the same direction as the vehicle was traveling when the marks were left.

For our example we measured the distance of the sikid mark from the beginning to the end and found it to be 225 feet long.

Now the first formula: f = F/W where f = drag factor of the surface; F = the amount of force in pounds shown on the scale and W = the weight of the drag sled held vertically off the pavemnent using the same scale.

So example: let's say that the F = 8lbs andthe weight of the sledis 10lbs then the f (frag factor) = 0.8 which is what one would expect from a newly paved macadam road surface.

Now that we have the drag factor, what is the distance of the skid in feet. Let's for example say the skid mark is 2250 feeet long. Next how many tires were locked and skidding? If all four, you would see overlapping skids from all 4 tires. So, assuming overlapping skid marks the next formula is the "Minimum Speed Formula". Again assuming from our analysis that all 4 tires were locked and skidding.

S = 30 * D *f

Where D = Distance of Skid Mark

f = Drag Factpor

and 30 is a constant (just go with it)I can give you the derivation of the formula, but I won't.

So, 30 * 225 * .8 = 5400

The square root of 5400 = 73.5 MPH

Therefore S = The minimum Speed of the Vehicle entering the skid is 73.5 MPH

To be fare the tires are rated for their breaking efficiency. The front tires are rated at 60% or 30% for each tire where as the rear tires are 40% or 20% for each tire. So if you have a vehicle with just the front tires and one rear tire skidding, rather than overlaping skids you have a breaking efficiency of 80%.

The symbol for Breaking Efficiency is (be)

If that is the case the formula changes: Where S = 30 * D * (f *be)

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MOMENTUM

Historically, we learn this from Newton:

In 1687 Isaac Newton first presented the three basic laws governing the motion of a particle. These laws gave insight into the effects of forces acting on bodies in motion. The three laws of motion are as follows: (1) Newton's first law of motion, the law of inertia, (2) Newton's second law of motion, the law of constant acceleration, (3) Newton's third law of motion, the law of momentum. Newton's First Law of Motion, the Law of Inertia A particle originally at rest, or moving in a straight line with a constant velocity, will remain in this state provided the particle is not subjected to an unbalanced force. The law of inertia describes the fundamental property of matter or a particle. Every object (body) remains in a state of rest or of uniform motion in a straight line unless acted upon by outside forces. This law states that motion is as natural a condition as rest. Just as an object at rest is in equilibrium, so is an object moving in a straight line at a constant speed in equilibrium.

This is a Physics platform, not accident reonstruction, so the last item for your enjoyment is the principal of "conservation of linear momentum" Let me give you an example: let's say for whatever reason you wanted to find the speed of a cue ball when it hits another ball on a pool table.

Both balls would seperate after impact and eventually stop their respective motions. Further it would have to be a very big table so the balls would not touch the rails. In the conservation of linear momntum, the physical restraints are not usually present.

Example: Two vehicles operating is opposite directions when one of the vehicles cross the center of the road and strikes the other vehicle head on but slightly at an angle - not direct nose to nose impact. One could determine the speeds of both vehicles at the time of impact. This is a little more complicated than sopeed from skid marks.

One has to determine the following to be able to give the exact speeds of the vehicles the information is:

Weight of Vehicle #1; Weight of Vehicle #2; The angle of approach; The departure angle and the final resting place of both vehicles. The following is an actual speed determination using this formula.

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