1. The problem statement, all variables and given/known data Pulley is mass less accelerating downward at 2.0 m/s2. M1 = 4.0 kg M2 = 2.0 kg M3 = 3.0 kg Find the acceleration of M2 & M3. 2. Relevant equations I made up the slope positive pointing up. Since M2 and M3 have the same acceleration, a2=a3 ⇒ a Since it's accelerating downwards, Apy = -2.0 m/s2 Mass 1: ΣF1y = m1a1y T-m1g = m1a1y T = m1a1y + m1g (1) Mass 2: ΣF2y = m1ay T-T1 = m2ay (2) Mass 3: ΣF3y = m3ay T1-m3g = m3ay (3) Pulley: apy = (a1y+ay) / 2 (4) 3. The attempt at a solution I added eq. (2) & (3) to get rid of T1 T-m3g = m2ay+m3ay (5) I plugged eq. (1) into (5) m1a1y + m1g - m3g = ay(m2+m3) Solve for a1y: a1y = (ay(m2+m3) - m1g + m3g) / m1 (6) Plug (6) into (4) apy = [ (ay(m2+m3) - m1g + m3g) / m1) + ay ] / 2 Solve for ay: ay = (m1g-m3g+2m1apy) / m2+m3+m1) = 2.87 m/s2 By the answer key, the acceleration for m2 and m3 is -2.87 m/22. How do I know that is negative and not positive? Is it because m2 + m3 have a higher mass than m1, thus when the pulley is accelerating downwards, m1's acceleration points up and m2 & m3's acceleration point down?