Falling chains as variable mass systems: theoretical model and   experimental analysis
  In the present paper we revisit, theoretical and experimentally, the fall of a folded U-chain and of a pile-chain. The model calculation implies the division of the whole system into two subsystems of variable mass, allowing to explore the role of tensional contact forces at the boundary of the subsystems. This justifies, for instance, that the folded U-chain falls faster than the acceleration due to the gravitational force. This result, which matches quite well with the experimental data independently of the type of chain, implies that the falling chain is well described by energy conservation. We verify that these conclusions are not observed for the pile-chain motion. 
C\'elia A. de Sousa, Paulo M. Gordo and Pedro Costa
physics.ed-ph