# Homework Help: Turning Acceleration into g's - Is 40 g's a lie?

1. Feb 3, 2010

### mrkern

Turning Acceleration into g's -- Is 40 g's a lie?

Col. John Stapp was a pioneer in human/g-force research. In one experiment he went from 0-632mph in less than 4 seconds, then at the end of the run, slowed to 0mph in less than 3 seconds. (You can see it in this video: ) Reportedly, he survived 40 g's of force, but our calculations show only 7 and 9.7 g's. Either we're wrong, or the claim is wrong/misunderstood.

3. The attempt at a solution

mph converted to ft/sec --> 632mi/hr * 5280 ft/mi * 1hr/3600sec = 927ft/sec

a= 927ft/sec / 3 sec(to stop) = 309ft/s^2

# of g's = 309ft/s^2 / 32ft/s^2 per g = 9.7 g's

If Col. Stapp did experience 40 g's, that would equal an acceleration of 1296ft/s^2.

dv/a= dt, so 927ft/sec / 1296 ft/s^2 = time to stop from 632mph in order to experience 40 g's = 0.72 seconds

So what's the deal? How was the original 40 g's calculated?

Last edited by a moderator: Sep 25, 2014
2. Feb 3, 2010

### PhanthomJay

Re: Turning Acceleration into g's -- Is 40 g's a lie?

Your calc is correct. The 40 g's is likely a misunderstanding of the concept of a 'g' force.

Note that if Col. Strapp weighs about 128 pounds (4 slugs worth of mass), then the force acting on him during his deceleration is (4)(309) = 1200 pounds or so, so mistakenly someone may have divided the force by 32 to come up with the '40' , which is the wrong approach, but I'm just guessing, I don't know what they did.....