Let the mass of each car be M.
v 44 ft/s
Conserve linear momentum:


Now, the frictional force on the sliding cars will be 2Mgμ where μ is the coefficient of kinetic friction and g=32 ft/s^{2} is the acceleration due to gravity. Therefore, the work which the frictional force does as the cars stop is


The cars begin (right after the collision) with an energy


and end with zero energy So,


so
. 
