# Homework Help: Deceleration falling into a harness

1. Oct 2, 2012

### wikithellama

Hi!

This isn't exactly a homework question - I am not studying physics (though I was good at high school physics once-upon-a-time) it is just something that I am interested in and Google isn't being very cooperative. If this is in the wrong place, please let me know - I thought since it is such simple equations it belongs in this section :)

I also may be over-complicating this somewhat and I am a complete n00b so please bear with me...

I am a Rope-Access tutor and it is "common knowledge" in the industry that a 600mm fall on a system with no shock-absorbing capabilities will generate a force of 6kN on the human body.

Now with the remnants of high school physics that I have, I believe that this would depend on the mass of the "victim" and on the elasticity of the harness etc... (I am thinking that I am in a little over my head)

I am not someone who takes things for granted so I am trying to work it out for myself.
I was going to ignore the elasticity of the harness, squishiness of the human body etc... but have found it isn't possible as I will end up with my time or distance during the impact being 0 which doesn't really work very well.

1. The problem statement, all variables and given/known data
2. Relevant equations
3. The attempt at a solution

Now I have managed to work out the velocity before impact for a falling person using
Vf2 = Vi2+2a*d using:
Vi = 0
a = 9.81ms2
d = 0.6m
and have a velocity of 3.431ms

I was going to find the deceleration using either the formula Vf=Vi+a*t or Vf2=Vi2+2a*d then use F=ma to find the force but I have realised that I have no clue what the distance or time taken to decelerate may be.

So my questions are:

Is there another way to work this out?
Has anyone come across a time or distance for the deceleration of the human body crashing into something (I think ignoring the harness for now is the best idea)?
Does it depend on the mass of the victim?
Am I actually on the right track?

My boss is using the equation E=mgh to work out the potential energy of the fall. He is just moving the decimal place to change it to kN ... is this a correct way of working out the force on the body?

Thank you very much in advance!

(I need some help with questions on vector mathematics at some point as well! :) )

2. Oct 2, 2012

### azizlwl

Average force,
$\bar{F} =\frac {\Delta(mv)}{\Delta(t)}$

I think the time taken to a halt depends on the flexibility of the body.