1. The problem statement, all variables and given/known data A stack of books rests on a level frictionless surface. A force F acts on the stack, and it accelerates at 4 m/s2. A 9 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.5 m/s2. What was the mass in kg of the original stack? 2. Relevant equations KE = 1/2mv^2 W = FD 3. The attempt at a solution I don't even know where to begin with this problem. Neither of the two equations is doable because I'm lacking necessary information, and that's all that's in the textbook except for Hooke's Law.