(4) Friction in Machines (5 pts) Rappelling is a technique to descend a steep rock face by sliding down a rope. A J-rack is a device used by rappellers to attach themselves to the descent rope. The rappeller is attached to the J-rack, and the J-rack "grabs" the rope (a). A basic J-rack is shown in (b) and (c). Describe how friction works to "grab" the rope AB. To A Bars Frame C J-Rack D D ( b ) EX 7.2.4b D To harness attached to rappelers (a) (c)