1. The problem statement, all variables and given/known data A 12 kg box slides across the floor with an initial velocity of +8.4 m/s. It comes to a stop after sliding +13.8 m. How much friction was present? 2. Relevant equations f=ma v^2=v(o)^2+2ad 3. The attempt at a solution I did the ''v^2" equation to find out what the acceleration was, so i could find out what F equaled in f=ma. I got -2.55=a. I plugged that into F=ma, F=877(-2.55), which then F=-2242.06. But can a force be negative? And also from there I'm completely lost.