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Gear ratings - a physics question 23-Nov-2007 At 1:44:06 PM das_blob

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>Something that has been bugging me for a while now is that gear is rated
>in kN. BUT, kN is a measurement of force so whether a climber has fallen
>1 m or 100 m he is going to exert the same force on the equipment (f=ma).
>But, the further a climber falls the more energy he accumulates so in my
>mind gear has to has to be able to withstand a certain amount of energy,
>so i thought it would be rated in kJ.

The force that is exerted on the gear depends on the negative acceleration and the mass of the climber (F=ma). If a 100kg climber falls, say, 10 m then she has kinetical energy of 10 kJ, corresponding to a velocity of ~15 m/s (where the mass of the climber does not play a role for the velocity). In order to deaccelerate from this velocity to zero over a length of, say, 1 m (depending on the rope stretch, for example), we need to apply a force of 22.5 kN on the gear - a normal biner would break (whereas 1m of rope stretch is not much for a 10m fall but it's only to make the point).

From a physicists point of view, it does not make sense to rate the gear in Joule. That would imply that it does not play a role how you add the energy to the system. In reality, however, that makes all the difference - imagine, e.g., to add 10kJ by heating: If the biner weighs 10g and is made of water (which is, strictly speaking, not true) you have to heat it to 240 deg Celsius to apply the same energy.