When we force protons through a narrow slit, we are gaining certainty in their position. As we gain certainty in their position, what should happen to the certainty on momentum?

Answer 1

Google Heisenberg uncertainty principle

certainty of product of position and momentum is limited so the more accurately you know position the less accurately you know momentum

Answer 2

eg:
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-007-electromagnetic-energy-from-motors-to-lasers-spring-2011/lecture-notes/MIT6_007S11_lec38.pdf