1. The problem statement, all variables and given/known data Our teacher said we can NEVER do an F=ma problem from an accelerating, or noninertial frame. (He said there are ways to do it, but we can not do it in his class), and I'm confused becuase often times he makes the "system" or makes a "free-body diagram" around an accelerating object. Today he showed as an easy problem, but I thought it uses an accelerating frame. Here is the problem: There is an elevator and there is a bathroom scale with a block with mass 3kg that sits on top of the scale on the bottom of the elevator. The scale reads 40N. There is another scale (a fish scale) that is attached from the ceiling to the block and that reads 20N. Find the magnitude and direction of the elevators acceleration. 2. Relevant equations F=ma 3. The attempt at a solution F=ma choose positive y in coordinate system to go downward -FT+Fg-FN=ma -FT+mg-FN=ma a=(mg-FT-FN)/m plug in values, ((3)(10)-20-40)/3=-10m/s/s.... (we assumed gravity is 10 downward), b/c C.S. is going downward the elevator is accelearing upward at 10 m/s/s. but isnt hte mass accelerating? how can that be the system?