1. The problem statement, all variables and given/known data Block C (m = 4 kg) sits on a frictionless horizontal surface. Block B (of m = 2 kg) sits on top of block C, and is attached to a rope that runs over the massless pulley as shown in the figure. Block A (m = 1 kg) hangs vertically from the rope. With what horizontal force F must you push Block C so that block A rises with an upward acceleration of a = 3 m/s2? All surfaces are frictionless. (a) 12 N (b) 47 N (c) 59.8 N [itex]\leftarrow[/itex] This is apparently the correct answer (d) 15.0 N (e) None of the above Figure: http://www.freeimagehosting.net/t/28bdd.jpg 2. Relevant equations Newton's Laws 3. The attempt at a solution (Viewed as a system) F = (mA + mB + mC)a F = (1 kg + 2 kg + 4 kg)a F = (7 kg)a [itex]\leftarrow[/itex] I must find this acceleration to calculate the force (From FBD of Block A - choosing UP as the positive direction) mAaa = T - mAg [itex]\leftarrow[/itex] Here, I am using aa as the 3 m/s2 acceleration of the two smaller blocks T = mAaa + mAg T = (1 kg)(3 m/s2) + (1 kg)(9.8 m/s2) T = 12.8 N (From FBD of Block B - choosing RIGHT as the positive direction) mB(a - aa) = T a = (T + mBaa) / mB a = (12.8 N + 6 N) / 2 kg a = 9.4 m/s2 ... Finally, plugging the acceleration into our first equation to find the force: F = (7 kg)(9.4 m/s2) F = 65.8 N If 65.8 is not actually the correct answer, what am I doing wrong in my solution? Thank you.