Question about the force of a falling object.

  • Thread starter PCruise
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  • #1
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I'm a theatrical lighting designer wear a fall arrest harness while climbing in trussing.
I can't find a calculation method for determining the weight put on the anchor point in the event of a fall.

I weight 160 lbs. my lanyard is 6 feet long. It has no shock absorber so I would come to a very sudden and complete stop.

Is there a method of calculating the weight of the shock load of arresting my fall?

Thanks,
Cruise
 

Answers and Replies

  • #2
Mk
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Sorry Cruise, I realized my first response was not an answer to the question you were asking. I said that the force that the ceiling anchor point puts on you is equal and opposite to the constant force of your weight. This is true, but only after you were hanging at rest.

Impact usually requires us to know how fast your change in velocity was, or how long it took you to come to a stop. Everything is actually a bit of a spring, elastic and soft.

FredGarvin said:
Impact equations are not an easy thing to figure out. The defining factor is how long does it take the object to come to a complete rest once it hits the stationary object. In theory you can treat it as a perfectly elastic collision and use momentum and go about your merry way. Real life does not involve perfectly elastic collisions. There is a finite, however usually quite large deceleration of the object when it hits the ground. It will all depend on how much of a distance the object is going to decelerate. The longer the distance, the less the deceleration and thus the less the force that is a result.

If you were to drop something on to a hard surface that was instrumented with an accelerometer, you would see that the peak acceleration will be much more that of gravity. That is the real force experienced in the drop. The curve would show you the impulse the object sees. Impulse is what you need to look into.
If the time it took you to stop was 0 seconds, that would lead to a calculation of infinite shock.

Welcome to PF!
 
Last edited:
  • #3
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The maths behind this type of calculation is very complex, way beyond me. But for practical purposes it can be simplified to:
- the weight of the object (you and your gear)
- multiplied by the acceleration distance (your free-fall) and then
- divided by the deceleration distance (arrest distance)

Arrest Force = Weight x Free-Fall Distance/ Arrest Distance

Basically if you try arrest a fall without a shock absorber decelerating you gradually you will generate very high forces. Easily enough to break your lanyard.... then you will just keep falling, regardless of anchor point strength. Generally this is considered as "not good". So get a shock absorber before you do anything!

Also, you should ensure that any anchor point you design complies with your relevant national standards. Also please ensure you have sufficient fall clearance to fall into.
 
  • #4
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Thank you, MK.
I didn't realize how essential the deceleration time is for the equation.
Which would be information I'd have to get from a rigging company, I suppose.

Thank you as well John.
I'm not concerned about the Lanyard breaking.
The harness and lanyard are manufactured by SAPSIS rigging for this purpose.
but you're correct about the fall clearance.
I'm working at 15-35' hieghts, which is why I didn't purchase the Shock Absorbtion lanyard. (it would add too much lenght for the necessary safety clearance)

Thanks agian!
Cruise
 

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