6 : Friction
The examples given so far have ignored friction to provide a simpler explanation of the force applied wherever a loaded rope is bent over an anchor .
Nevertheless , even the most efficient ( low friction ) pulley will invariably apply some friction . Friction is both the best friend and the worst enemy of rescuers , depending on whether the casualty must be lowered or lifted .
Wherever friction occurs in a system , it will reduce the force being carried by the rope from that point on .
In the image on the right , a casualty weighing 100kg is hanging on the rope running to the left .
Friction is applied by a turn around a tree branch , which reduces the load being carried by the rope throughout the system from that point on ( orange arrow ). The casualty ’ s weight can be held with a comparatively light belay of 0.2kN . It is important to remember however , that the original load in the rope must go somewhere – in this case it is being taken by the branch .
It is also important to note that the direction and magnitude of the resultant force applied to the branch is different to that which would be applied if a low-friction pulley were to be used here .
© Arboriculture Australia 2022 - 123 -