Apparent momentum violation when pucks stop due to friction

[raman911]

Homework Statement

We know that momentum is supposed to be conserved in this collision. We observe, however, that the pucks eventually come to a stop, at which point they have zero momentum. Give a solid explanation (using the law of conservation of momentum) for this apparent violation of the law of conservation of momentum.

 

[G01]

What causes a change in momentum?