A pulley and block system is a favorite type of system for analysis when considering work-energy problems in beginning dynamics. It's commonly recommended that the best approach to the system is to write an equation for the whole system, while ignoring the work done by cables/ropes and focusing on the blocks themselves. What is the property of rope and pulley systems that allows the analysis to ignore them in this case? I'm just curious and am looking for insight. Thank you for your time.