# Re: crash tests

```does anyone feel like tutoring, i have a physics final on Wednesday :).

Lawrence C Leung wrote:

>         I wasn't referring to the Energy of the crash. I was referring to the
> Appearance to the driver. I was trying to make the point that many people
> think that they are going into accidents at rates of speed much greater
> than their actual impact speed so I was trying to STRESS what the
> impending crash would LOOK like to the driver. Most drivers really
> inflate their accident speed, so they think that their car was better at
> survival than what was indicated in the NHTSA crash tests. Since MOST
> drivers, upon seeing that they are about to crash will, at the least,
> lock the brakes (even then there is some braking, actually more than you
> think) they tend to crash at speeds much slower than the 55-65 MPH that
> they thought they were doing as they crashed. They forget the braking
> part. So, a 30 MPH header is actually more valid than it appears.
>
>         Now, as for the energy of the crash, the only sane (i.e. reasonable
> cost, wrecking only one car) way to approximate the energy of two cars
> colliding into each other at 30 is to run a car into a fixed barrier (or
> fully loaded semi, if the trailer moves, it's motion absorbs energy,
> invalidating the results). Here, Consevation of Momemtum suggests that
> the immovable nature of the barrier means little energy absorbtion by the
> barrier, (momemtum of barrier = 0 before and after the crash) all of the
> energy is absorbed by the crashing car, which is why the enegy effect on
> the crashed car is doubled, approximating the effect of a head on crash
> of a car into a like size car travelling at the same speed as our test
> car.
>
> BTW  (the total energy of one car doing 60 crashing into a barrier is 4
> times the energy of one car doing 30 into the same barrier, from KE  =
> 1/2 mv^2). Thus, for two cars doing 30 head on, the energy is only double
> that of the single car doing 30. So, the most severe crash in this list
> is the 60 MPH one, not the double 30 one. THIS is why it hurts more to
> strike a tree rather than another vehicle. Not that you're likely to be
> happy with any of these results.
>
> .. Be aware that two cars travelling towards each
> >other,
> >each at 30 MPH is an extremely severe collision, visually, it would
> >LOOK<<<<<<<<<<<<<<<<<<<<<<<<< (Note this emphasis, LOOK not SAME effect)
> >like driving into a parked car while you were doing 60 MPH, brakes
> >OFF.
> >
> >I believe physics shows us that 2 cars head-on each going 30mph is not
> >the
> >same as 1 car going 60 into a parked car.  Each car absorbs 30mph, not
> >1
> >absorbing none (unless it is a semi-truck) and the other car absorbing
> >all.
> >Small point, but a common misconception.
> >
> >
> >Tom E. Thomas
> >Data and Video Network Engineer
> >BadgerNet | BTM | DTM | DOA | State of Wisconsin
> >tom.thomas@doa.state.wi.us
> >

```