1. The problem statement, all variables and given/known data A block with m=20kg is sliding on top of a block of M=10 kg that is sliding down an inclined plane with 30 degrees to the horizontal. All surfaces are frictionless. Calculate the acceleration of the blocks. 2. Relevant equations Newtons second law: [tex] F⃗ net=ΣF⃗ =ma⃗ [/tex] 3. The attempt at a solution For sake of generality, let's say that the angle is indefinite. I have sketched a free-body diagram and [tex] F⃗ net=-mgsin(\theta)-Mgsin(\theta)=(m+M)a⃗ [/tex] since the boxes will moves as one unit. Solving for acceleration gives [tex] a⃗=-gsin(\theta) [/tex] which is wrong. I think it is something with the signs. They both have the same sign since they move in the same direction(downward=negative) but even if both positive would yield wrong answer.