I teach a first year Physics course. The incoming students (grades 10-12) have little to no Physics background. We are making some modifications to our work and energy unit. One modification that has been discussed is what to do about kinetic friction. At present, we account for friction as a negative change in mechanical energy by calculating a pseudo-work term (-fkΔx). We don't look at examples such as a car peeling out from a stop sign. Again, this is a 1st year Physics course... We have considered changing our accounting of energy changes due to friction as a change in internal energy. For the examples we consider, we would state that ΔEinternal = fkΔx in the absence of thermal energy transfer in / out of the system. However, should we include the surface in the system as well? Is this getting to be a little too much for the 1st year student? I'm wondering what all of you do about this. Thanks.