posted by Tammy .
A stack of books rests on a level perfectly smooth surface. A force F acts on the stack and it acclerated at 3m/s^2. A 1kg book is added to the stack. The same force is applied and now the stack accelerates at 2m/s^2. What was the mass of the original stack?
I think i would use F=ma and F=m_total*a
I chose 5N to be the applied force. so F=ma
But this isn't valid. What am I doing wrong?