1. The problem statement, all variables and given/known data FInd acceleration and tension. Take g=10m/s^2 2. The attempt at a solution By drawing the free body diagrams of every block and simultaneously solving all the equations, I got the answer acceleration= 30/7 m/s^2, which is also correct. But when I try to treat all the pulleys and blocks as one system to get the acceleration, I get a different answer. f=ma ⇒ 40g+10g= 70 * a (since weight of 40 kg block and 10 kg block are the only forces acting downwards) ⇒ a=500/70 m/s^2 Somebody told me that I should have rather subtracted 40g and 30 g, so that would be 40g-10g=70a ⇒a=30/7 which happens to be the right answer. But I don't understand why will we subtract the force vectors if they are all acting in the same direction ;ie, downwards. Where am I wrong with the second approach?