The Traffic Accident Reconstruction Origin -ARnews-

Re: speed from skid

Ed Phillips (edphill@aol.com)
Thu, 1 May 1997 09:16:55 -0400 (EDT)

Mike, I know this can seem a little confusing at first. We have a few ways to analyze these types of events. Two of the ways are energy and momentum. They look alike on paper, but so do houses and horses. In reality they are very different. In a skid event, we are computing the loss of kinetic energy, not momentum. A fundamental theorem of energy is the work-energy threorem. This theorem tells us that Work (F*d; where F = m*a) is equivalent to the change in kinetic energy. Essentially, the transfer of KE to some other form. KE = 1/2*m*v^2. Setting the two equal we see that m*a*d = 1/2*m*v^2. As the mass (m) is on both sides we may divide by m and anything divided by itself becomes one. If we then multiply both sides by two we get; v^2 = 2ad. When we arrange the solution in units of mph it reduces to s^2 = 30*d*f. The key to the energy loss is how the unit resisits slowing (a or specifically f) and the distance traveled. For units that will travel the same distance and
for the same resisitance, their speed differential (loss) is the same. As they slide they transform their energy of motion into some other form of energy, primarily heating up the tires and roadway, and air around the tires. So a VW Beatle and a Cadillac will stop in the same distance if they have the same reisitance (rate of slowing). But, heavy trucks don't slow at the same rate as cars, so they tend to travel greater distances at lower friction rates for the same speed change. Momentum is interested in how objects change direction and speed when interacting with other objects. Kinetic Energy is a look at how the motion energy changes to other forms of energy, heat loss, frictional losses, damage, etc. If I'm not being clear, e-m me and we can discuss it in more detail.

Ed
Ed Phillips
edphill@aol.com

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