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PLease help with calculation Safety harness makes a person stop

  1. Sep 10, 2013 #1
    I need to know the amount of force placed on the back/spine in the following scenario.

    A 210 pound person is in a free fall for 12" when a safety harness snatches them to a stop. The safety harness is secured to a tripod rig system of ropes and pulleys. What is the amount of pressure or force exerted on their back where the harness strap is located? (The strap in question goes across the shoulder blades)
     
  2. jcsd
  3. Sep 10, 2013 #2
    not enough info
     
  4. Sep 10, 2013 #3

    Nugatory

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    You'll have to specify how much stretch there is in the harness and its supporting ropes and pulleys. For example, if there's no stretch at all, the force will be infinite (we have to bring the person to rest over zero distance and hence in zero time).

    When you do allow for the stretch, you'll be able to get a pretty decent approximation from ##W=Fd## and ##E=mgh## - the work done by the force in the harness pulling on the person for the stretch distance must be equal to the energy the person gained from the fall.

    If you're trying to design a real safety harness not just doing a thought experiment, don't ask PhysicsForums, ask a pro.
     
  5. Sep 11, 2013 #4
    I appreciate the help. Here is some more information....
    Not trying to design anything...I got hurt on the job with the above scenario. Have no clue how to figure out just how much force was put on my back when harness snatched me. There was no give/stretch in the harness. There is stretch in the ropes. Although I don't know exactly how much, I do know from handling them, etc. that there is not much. I don't need an exact amount, but a close estimate will work. Thank you!!
     
  6. Sep 11, 2013 #5
    It is impossible to give any estimate without an estimate of the stretch in the rope, and only you know what sort of rope you were attached with. To be absolutely clear, if there is "no" stretch in the rope, then the force is infinite. Actually a "tripod system of ropes and pulleys" sounds like a proprietary system such as ManSafe (in the UK). If this is the case you can look up the load parameters (when used according to specification), however if there is no "give" at all in the system it sounds like a so-called fall restraint system which is designed to prevent you from getting near enough to an unprotected edge to fall, not to protect you from injury in a fall. If it is a fall restraint system and you did in fact fall then either you were not using the system correctly or it was not specified/installed correctly.
     
  7. Sep 11, 2013 #6
    Note that 12" means 12 inches, do you mean that or do you mean 12 feet? You say that the injury was from the shoulder strap, does that mean you were upside down? The physics of this are not really relevant anyway, the full circumstances of the incident should be examined by a health and safety specialist (if this is construction work in the UK this is probably a legal duty on the Principal Contractor).
     
  8. Sep 11, 2013 #7
    I was dropped 12 inches. It's in the US. It was a full body harness, so I was completely vertical during the free fall. I was using it corretly...the person that was supposoed to be my "spotter" and lower me down into the confined space for me to rescue the victim had slack in the rope, so when I went over the edge of the confined space entry point, I fell.
     
  9. Sep 11, 2013 #8
    Gotcha. Have you referred to the method statement applicable to this rescue? Does it require zero slack (i.e. positive tension) to be maintained by the spotter at all times?
     
  10. Sep 11, 2013 #9
    In order to prevent accidents like this from happening, tension must be maintained on the lines at all times. Had there not been slack in the rope, I could not have fallen.
     
  11. Sep 11, 2013 #10

    sophiecentaur

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    The 'Physics' of this situation is not as relevant as the degree in which any regulations were or were not followed. It seems to me that the 12 inches in questionis the only figure needed. If that is regarded as adequate by the regs then you have no case. If it is not adequate protection and there is strong evidence about the actual distance fallen then you do have a case. A good lawyer will do you more good than any amount of 'amateur' knowledge of Science. Insurance claims operate in a different Universe.
     
  12. Sep 12, 2013 #11
    I know that I have a case and I've retained a lawyer. I wanted to know the amount of force that was applied to my back when I fell.
     
  13. Sep 12, 2013 #12

    russ_watters

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    The question is impossible to answer without exact knowledge of the setup, but I will say that 12" is an insignificant distance that would be virtually impossible to cause injury that you couldn't cause yourself by jumping up and down.
     
  14. Sep 12, 2013 #13

    A.T.

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    Tie a hangman's noose around you neck and drop 12". By your logic It can't be worse than jumping 12".

    Our legs are build to dampen impacts. Our back not so much. But I agree that it too complex. It depends much on the elasticity of the rope and how the harness distributes the force over the body.
     
  15. Sep 12, 2013 #14
    Well, I have 4 bulging disc and a pinched nerve to prove it is a significant distance. Don't know of anyone that can cause that by jumping up and down.
     
  16. Sep 12, 2013 #15

    russ_watters

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    Or a hook through your nipple!

    C'mon, A.T., be reasonable. He was not suspended by his neck or his ear or his eyelid, he was suspended by a safety harness. The distance involved makes the impact roughly equivalent to sitting down too fast into a hard-bottom chair (and substantially less than falling backwards and landing on your butt). And by the way, sitting or standing (both the transitions and the static acts of being in those positions) are a common causes of back injury. The acts of sitting down and standing back up involve poor posture which puts strain on your back. As can sitting itself.

    Then you need to talk to more people, most especially a doctor. Back injuries do not require impacts. My mother got her herniated disc and pinched nerve (and following decades of debilitating problems) by bending down to retrieve a can of soup she dropped on the floor at the supermarket. The reality is that over a lifetime of use, the back gets weak and prone to injured even in normal use.

    But in any case, the speed involved here is so low (about 6 feet per second - a brisk walking speed) that your body can absorb even an instant halt with its own internal deformation.

    I'm sorry you hurt your back. Getting old sucks. But I don't see how there can be any blame for anyone else than God here.

    That said, our society is so litigious that it doesn't seem to recognize true accidents. So all you likely need to prove is that they didn't follow procedure, not that it was a bad fall that should be expected to cause injury.
     
  17. Sep 12, 2013 #16
    I understand that back injuries do not require impacts, however, I had one and now my back is hurt. I'm simply trying to figure out is there is a way to calculate how many pounds of force were placed on my back when I was dropped. There was no give in the ropes or harness.

    Just so you'll know - this is the first time I've ever talked with a lawyer for any reason. It's not God's fault my spotter left slack in the rope and dropped me.

    I get in and out of chairs every day. I don't know how you do this, but the impact I had that day was nowhere near what I do numerous times a day getting in and out of a chair.
     
  18. Sep 12, 2013 #17
    In a way Russ is correct. It is not the 12 inch drop that cause or you back injury, even thought that is the prime motivator, but the position, angle, orientation, twist of you body however you want to call it, when the rope and harness stopped your fall that was the contributor to the injury. You probably woul have not suffered any ill effects by being dropped some 86 times ( pick a number ) before, but in this one case an unknown factor(), led to an injury. I often tell people who say they hurt their back by lifting a heavy object, that one can hurt their back by picking up a penny, just by doing the movement the wrong way.

    There is always some give in the rope and harness, in your setup perhaps slight, so the force cannot be determined until that is known.

    Do people who parachute out of a plane have the same sort of harness as yours. One of the training procedures for combat parachutists was, or still is, to climb onto a platfrom some 12 feet up or so,( maybe some body who has done the training can come up with a more accurate height of the platform ) harness youself up, and jump off, with the harness breaking you fall a foot or so above the ground. You would thus experience a simulation of the opening of the parachute and the impact that had upon your body before you jumped out of a plane. I can imagine if you body is horizontal rather than vertical the effects would have a somewhat different outcome.
     
  19. Sep 12, 2013 #18

    CWatters

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    Hi Adam,

    I've only ever done a bit of indoor climbing and that was years ago.

    I'm wondering if there was something unusual about the set up? I think some types of rope do have lower stretch than others. The use of a block and tackle would also make the stop more sudden. It sound's like a combination of factors might have caused the problem.

    Sorry but as others have said there is no way we could calculate or even reliably estimate the actual force unless you have data on the rope and the amount of slack. At issue is the stopping distance. You might have fallen 12" but what matters is the distance over which you stopped falling. That depends on the properties of the rope and what it's connected to and other unknowns. If you try and use a stopping distance of zero you get nonsense answers such as infinity. If you fell say 10" and stopped in say 2" you might experience 5g. But it's impossible to calculate how fast your body would have stopped - not all of it will stop at the same rate. There will be some give in the harness your body etc.

    I suspect the best way to get an answer would be to use a test load/dummy or even a fit volunteer and run the experiment using the kit you were using.
     
  20. Sep 12, 2013 #19

    sophiecentaur

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    You are not getting the message here. The force will depend on the precise set up and the characteristics of the webbing. You could fall 12" under constant braking force (low) and come to a halt with no injury. You could fall 12" onto concrete and crack your skull open. Both scenarios are covered by your description. Your case cannot be based on the inadequate Physics provided. Your lawyer would surely know that???
     
  21. Sep 15, 2013 #20

    cjl

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    There is no such thing as a harness with no give, and the amount of force placed upon you by the harness depends very strongly on the exact amount of give that the rope and harness have. If they are not rated for shock load, and are meant to be kept under tension at all times (which it sounds like is the case here), then the amount of give could be very small, but it is nonzero (and important in figuring out the forces here). Generally, ropes designed to arrest a fall (and not just support a static weight) have a much larger give than ropes designed for use under tension at all times, and that helps minimize impact forces.
     
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