# Physics expert needed **URL:** \ **Category:** Factual Questions\ **Created:** [December 2, 2009, 10:07am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689 "2009-12-02T10:07:26Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![bengangmo](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@bengangmo](https://boards.straightdope.com/u/bengangmo)\ **Post date:** [December 2, 2009, 10:07am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/1 "2009-12-02T10:07:26Z")
Need answer fast… To propel an 18kg (5 yr old) pedestrian 8m what is your “best estimate” of car speed? ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [December 2, 2009, 10:09am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/2 "2009-12-02T10:09:02Z")
I hope this isn’t something you’re planning… ---
**Author:** ![bengangmo](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@bengangmo](https://boards.straightdope.com/u/bengangmo)\ **Post date:** [December 2, 2009, 10:13am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/3 "2009-12-02T10:13:06Z")
nope…real case being discussed on another board Some people saying that to send pedestrian 8m confirmed speeding - I have my doubts, just wondering if there is anyway to at least guesstimate the speed from the distance (I know too many variables to give truely accurate answer) ---
**Author:** ![\_cc](https://avatars.discourse-cdn.com/v4/letter/_/e19b73/32.png) [@\_cc](https://boards.straightdope.com/u/_cc)\ **Post date:** [December 2, 2009, 10:27am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/4 "2009-12-02T10:27:13Z")
At what angle was the victim launched? Most impacts are going to knock the person down who will then roll/slide to a stop, the distance at which they do so greatly determined by a variety of factors. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [December 2, 2009, 11:27am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/5 "2009-12-02T11:27:16Z")
Have a physicist do that calculation, and you’ll end up with a suffocated five year old skidding at uniform speed across an infinite frictionless surface… Seriously, though, without some detailed modelling I can’t see any way to obtain a meaningful answer that’s not completely swamped by its error bars. The key sticking point will probably be how much momentum is actually transferred in the collision – which will depend on the point of impact, material constants (energy that goes into deformation, sound), angle of impact etc. To not be completely unhelpful, though, [here’s a paper](http://www.aprosys.com/Documents/40_L_Martinez_final.pdf) (PDF) outlining what goes into the simulation of such accidents, and [here’s another one](http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V3K-4V17CP2-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1118736647&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8fab6eb7f98c05e5a6d756b9b37c4983) explicitly concerned with obtaining the vehicle speed, though you’d have to pay for the full article. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 2, 2009, 12:06pm UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/6 "2009-12-02T12:06:45Z")
Did the victim go 8m on the fly, or end up at 8m after rolling? Not that I have an answer for you, but that will change the answer. This isn’t really a physics problem, it’s one you’d have to find empirical data for. ---
**Author:** ![bengangmo](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@bengangmo](https://boards.straightdope.com/u/bengangmo)\ **Post date:** [December 3, 2009, 1:03am UTC](https://boards.straightdope.com/t/physics-expert-needed/519689/7 "2009-12-03T01:03:26Z")
Hmm…I can’t really answer the questions posed, thanks Half Wit (tee hee thats fun to say) I will take a look at the papers. Cheers