Three objects with masses m1 = 66.0 kg, m2 = 37.4 kg, and m3 = 21.8 kg are hanging from ropes that run over pulleys. What is the acceleration of m1? (Take the upward direction to be positive.) So for the two outside blocks, I know that the forces acting on the blocks are T1=m2g+m2a, and T2=m3g+m2a But why in literally every break down of this problem I see, the force acting on the main block is m1g - T1-T2 = m1a, but not T1+T2-m1g=m1a? I can't seem to get the answer when I do it with the latter force layout because I end up with a negative not belonging somewhere.