Forces in Lifelines

Lifelines are exposed to forces hundreds of times greater than the force of a falling person. It should be designed by a competent person.

ZIPPY’S tug-of-war game – How strong are you?

The idea is to raise a small cast-iron weight with a mass of 20 kg high enough to place it on a counter that is 860mm high. The rules are simple:

  • Only two persons may participate using a short length of sturdy rope.
  • The cast-iron weight must be lifted up whilst it is hanging in the middle.
  • Raising the weight by only pulling horizontally on the line.  
  • Their hands may not be raised any higher than where they were at the start. 
  • No jerking is allowed

At the start of the game the two persons hold the weight up off the ground with their arms out in front of them with their hands 400mm apart. The height to the underside of the weight is measured. See Figure 1

Figure 1

The forces in each rope with the two ends being held 400mm apart amounts to 11kg which is not much more than if the ropes were to hang vertically downwards. The weight is 350mm above the floor

Now they move away from each other gradually as far as possible to see if they can lift the weight high enough to place it on the counter.

The result – FAIL – The force required is too much even for the strongest persons!

If the distance is increased to 1130mm the force in each rope increases to 39kg, the weight is now 748mm above the floor.

Figure 2

If the distance increases by 22mm (1130 to 1152) the force increases to 58 kg and the height increases to 798mm, but it needs to reach 860mm.

Figure 3

So now they increase the distance to 1166mm – just 14mm more but the force increases to 115kg per person and the weight is now 849mm off the floor but still 11mm too low! Can they lift it more? Each person needs to pull with a force of about 170kg to raise the weight just another 11mm!

Figure 4

 Zippy saysNo – two persons cannot stand on their feet and pull so hard – they won’t even be able to pull much more than 40kg before falling over”   

This is the proof that lifelines are exposed to forces hundreds of times greater than the force of a falling person. Get an engineer to design the line and its anchors.

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