Mass m1 is attached to a rope that goes over an ideal pulley. At the other end of the rope is another pulley. A rope goes over this pulley connecting two masses m2 and m3. Find the acceletation of the masses and the tensions in the ropes. When considering the first mass, I pretend that masses m2 and m3 are one mass and I call it M, so <--m1*g---(m1)---T1--> <--Mg-----(M)----T1--> am1=-aM -m1*g+T=m1*a -Mg+T=-Ma I substract the two equations and get, am1=g(m1-M)/(M+m1) At this point I stop, because the book already says I'm wrong. The book get a=g(4m2m3-m1m2-m1m3)/(4m2m3+m1m2+m1m3). What did I do wrong?