# Physics expert needed

To propel an 18kg (5 yr old) pedestrian 8m what is your “best estimate” of car speed?

I hope this isn’t something you’re planning…

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)

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.

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 (PDF) outlining what goes into the simulation of such accidents, and here’s another one explicitly concerned with obtaining the vehicle speed, though you’d have to pay for the full article.

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.

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