Free-Body Diagrams: Several Objects and Newton's Third Law

1. Feb 23, 2010

stridle

1. The problem statement, all variables and given/known data
A box of mass m2=3.5 kg rests on a frictionless horizontal shelf and is attached by strings to bodes of masses m1 = 1.5 kg and m3 = 2.5kg. Both pulleys are frictionless and massless. The system is released from rest. After it is released, find (A) the acceleration of each of the boxes, and (B) the tension in each string.
T1=M1*g-M1*accel
T2=M2*g-M3g-M3*accel
Fn=M2*G
-T1+T2 = M3*accel

2. Relevant equations
Summation of Force in the x direction = m * acceleration. I set up three of these equations one for each object. I also set up the equation summation of force in the y direction = m * acceleration for m2 on a horizontal shelf.

3. The attempt at a solution
I have attempted to find ways of plugging the different equations into the other equations to produce the acceleration, but have failed to get the answer the textbook gets. Which is 1.3.

Last edited: Feb 23, 2010
2. Feb 23, 2010

stridle

I figured it out. My T1 equation has reversed signs! Silly me. Thanks!

3. Feb 23, 2010

stridle

Actually I cannot figure out where I went wrong on the T1 equation to mess up the signs. Do I need to acknowledge that M1 will be the weight going up and M3 will be in the downward direction? I think that will give me the correct signs for T1? I am a little confused I must admit.