weight x speed theory

Splash

New member
Okay, I've heard it too. Forever. 10 pounds, times 20 miles, equals 200 pounds.

Hmmm...

What about metric!?

10 lbs x 20 mph = 200 lbs

4.5 kg x 32 km/h = 144 kg = 317 lbs

Okay, I know the 'formula' doesn't match up exactly with mph and pounds... that would be *too* perfect. And I know it's a decent rule of thumb just to give people an idea... but really, IS there a true formula (boy, for a teacher I sure don't know my stuff, do I?) that equates this? And what IS the formula? Or is it really weight in pounds times speed in miles?
 
ADS

SafeDad

CPSDarren - Admin
Staff member
Yes-

Force = Mass * Acceleration

Acceleration = Change in Velocity / Change in Time


The problem is that we don't often know the exact amount of time it takes for an object to stop. Even a small change in the number used for the time can make a big difference in the force. That's why the approximation of weight*speed is used for simplicity. It really isn't a good approximation, but the real formula isn't any better if you don't know what to put in for the time...
 

Victorious4

Senior Community Member
Oh, I'm laughing sooooooo hard

Because my SO is a nut about physics & was really trying to understand why we just say "weight * speed" ... I put up with his physics lessons for a few weeks now :eek: Finally, after watching me do a seat check for one lady he got it that at least part of the reason we simplify it is so that the parents can easily understand & so that I can do the math in my head without looking like a fool, LOL :p :D
 

thepeach80

Senior Community Member
The actual formula for it is quite complicated. We actually had a lady in my tech class that worked for NHTSA and did crash site reviews and knew the real formula and actually used it. This is much easier! I wish there was an easy way in metric though, it would help some.
 

Jeanum

Admin - CPS Technician Emeritus
Staff member
Same here, DH was a physics major, so I get to hear his F = M * A "lecture" if he hears me phrase it as the simplified weight X speed. But using his physics background has been helpful for me to persuade him on the subjects of extended RF and harnessing. :)
 

Dreaming_of_Speed

Senior Community Member
It bothers me to use it (i did it earlier :) ) and i do know the physics formula but its a lot of algebra that i dont want to bog down a parent with. The weight timed speed is easy to use and easy for them to understand. depending on how specific you want to get i can throw in some calculus and make it more complicated but much more accurate. (i miss being in college where one actually did this!)
 

griffinmom

Moderator - CPST Instructor
weight * speed is only intended to be a rough estimation of crash forces. It is an oversimplified formula to get parents to understand what happens in a crash. It was never intended to be mathematically correct.
 

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