Replace the block with cable and the plate with conduit, and we have cable pulling . . . Neither the cable nor the conduit is flat. There may be more than one cable, which can result in complex rubbing surfaces. Pulls are not straight, so forces other than gravitational weight occur at conduit bends. Finally, pulling lubricants change and lower the friction coefficient.
Lubricants reduce the coefficient of friction, and thus the force required to pull the cable.
In practice, this can mean a reduction in tension of 35 to 95 percent, depending on conduit route and cable jacket type. Not only must the lubricant be slippery, but it also must be compatible with the cable jacket with no long term adverse effects. While oils and greases sound like fine cable pulling lubricants, they're not. These materials can swell and weaken the plastic jacket on the cable. Some of the wax and soap lubricants used on electrical cables are not suitable for communications cable, as they can stress crack polyethylene jacket.