Calculate Impact Speed of Car Hitting Sign

[Scott1965]

Hi, I hit a 4x4 post holding a streetsign up on a country road. I guess I was doing about 20 or 30 when I went off the road and struck the sign. It snapped the 4x4 in half at impact point and then toppled onto and almost into my windshield. I am looking for a physics formula to help accurately identify my speed. My Dad says I was going at least 50. I say no way. Help if you can. Thanks S

 

[flatmaster]

Well, you're looking for your speed when you struck the sign. What data do you actually have? All that you've posted so far is your best guess at your speed. What do you have that you could actually measure to calculate your best guess at your speed?

 

[pervect]

Google finds http://www.4n6xprt.com/Equation_Comparison-Frontal_Narrow_Object_Impacts.pdf

It was published in JN Cofone, AS Rich, JC Scott - Accident Reconstruction Journal, 2007

but I'm not particularly familiar with the journal or the literature on this topic. It "smells OK" though - I can't totally guarantee how well academically this journal is accepted.

It seems to suggest that your truck should have been pretty badly mangled. It also suggests that utility poles have been fractured in 30mph collisions, though it's not clear how thick they were. (The Atlantic city tests on pg 1, specifically).

 

[adamwho]

Hi, I hit a 4x4 post holding a streetsign up on a country road. I guess I was doing about 20 or 30 when I went off the road and struck the sign. It snapped the 4x4 in half at impact point and then toppled onto and almost into my windshield. I am looking for a physics formula to help accurately identify my speed. My Dad says I was going at least 50. I say no way. Help if you can. Thanks S

The 4x4 is organic material and not subject to rigorous analysis before you hit it so it is unlikely that you will find a nice formula.

There are many sources which you can consult about the average strength of a 4x4 post.

 

[berkeman]

Clearly some additional experimentation is warranted...