The Physics of Falling

Article by planetFear
Tuesday 26th May 2009

When placing gear, we usually have few options as to how we place it, with the rock largely dictating what we do with the protection we carry to keep us out of harm’s way.  Where there are several options for placing gear, an appreciation of how the gear will be loaded in the event of a fall by leader or second could make a big difference to its holding power.

With ‘straight-up’ climbs, we intuitively place protection to withstand a downward force. In fact, there is always a variable degree of outward and lateral (i.e. non-vertical) force on protection involved in any fall. This is most pronounced on very steep climbs and traverses. This article is concerned with the direction of that force, and its effect on the protection. With protection such as threads and bolts, the direction is irrelevant, but with nuts and cams, that direction is critical - after all, to remove them we simply alter the direction of force!

Have a look at these photos of the classic Pembroke E2 5b Lucky Strike. The second is simply a close-up shot of the same thing.

Taking the gear by the climber’s waist as an example, if the climber fell off onto this piece of protection, the forces would look something like this:

As you can see from the above diagram, the force on the top runner occurs at the intersection of the angle formed between the rope and the top runner. If the runner was placed only to withstand a downward direction of force, it could therefore be unseated and rip out.

Looking at a close-up of the photo, you can see that the wire placed next to the climber is perfectly suited to the direction that we anticipate. However, if the second were to fall off onto the same piece of gear, the direction of force would be entirely different – as the next diagram explains.

On a steep route, the gear would need to be placed to take an outward (as well as a downward) pull. Look carefully at this photo of the same climber taking air off The Butcher (E3 5c). Notice the initial direction of pull on the gear is outwards, which could unseat a poorly placed nut.

Summary

The diagrams above are clearly a simplification of the very complex three-dimensional physics of falling off. We have presented the direction of pull on the gear as being singular, when in fact, the actual force will by applied over a range of directions as the climber swings. In the following photographs of top UK climber Nic Sellers taking a seventy-foot whipper off the crux of Chupacabra, (an awesome E8 6c in Huntsman's Leap, Pembroke) it is easy to see the changing direction of force on the protection through the course of the fall. It is also worth noting that - in the case of falls longer than twenty feet on steep rock - the fall will not follow a linear trajectory but something like an arc. This means that the load placed on the gear will change significantly as the climber falls.

1) The load on the gear is at 90 degrees from the cliff at this point in the fall

2) As the ropes come tight, the 'arc' closes and a downward rather than outward load is placed on the gear. Photographs by Gavin Symonds

Even though this fall is long and spectacular, the steep nature of the climb and the perfect protection means it is, in fact, a safe one. The most important point to note from these photographs is the importance of anticipating the direction at which the gear you place will be loaded, and not to assume it simply needs to withstand a downwards pull – which paradoxically is the direction it is least likely to be pulled initially in a real fall. Wires should be placed to withstand a range of directions of pull, and placed deep in cracks where possible. It is also worth remembering that cams often have a far higher outward-pull holding ability than nuts and wires. If you are in a situation where an outward pullload is likely to be severe (i.e: on gear placed at the back of a roof) a good cam placement will typically be more secure than a wire.

Top trick: to ensure the gear you have placed is loaded at the ideal angle of pull (vertical), make sure that you extend all your runners properly so the rope follows as direct a line as possible. This is of particular importance on steep routes and climbs that follow an indirect line up a cliff, such as Lucky Strike (see above).

A secondary point is to always ensure you keep your second safe on a traverse. This can be achieved by placing gear that can withstand a multi-directional pull like threads, cams, and opposing wires. It also means placing good protection after the hard moves as well as before them. 

 

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