The collision of a field-driven polymer molecule with a stationary obstacle is a simply posed but non-trivial problem. Although analogous to a classical rope-and-pulley problem, this microscale problem is complicated by the polymer's elasticity and fluctuating configuration. Collision studies have been pursued to investigate polymer-polymer interactions and the size-separation of large DNA molecules. For sufficiently strong impacts, a coiled polymer hooks around the obstacle and unwinds into a hairpin configuration. In this talk we will discuss results from single molecule studies and Brownian dynamics simulations of a single DNA colliding with a single obstacle. We find that the previously hypothesized taut rope-on-a-pulley mechanism explains only a small a fraction of the collision events. We develop a micromechanical model which explains the other events and discuss the implications for DNA separation technologies.