Good for you.
Remember: Friction always does negative work. In fact, if an object rolls to a stop, then the amount of work done by friction against it is equal to the energy that it had at some point prior.
Also, since there are two objects involved in this collision, it may not be all that important that you acquire actual numbers as it may be the case that variables could drop out along the way.
Lastly, since we are talking about two different carts, you have considered that they might have different coefficients of friction, right?
And just so you don't feel so alone in this quest for mass, here's an article about the happenings at CERN and the quest to find the Higgs Boson!
