An inclined plane, a pulley, and three masses

[smashd]

Homework Statement

A system comprising blocks, a light frictionless pulley, a frictionless incline, and connecting ropes is shown. The 9 kg block accelerates downward when the system is released from rest. The acceleration of the system is closest to:

A.) 1.9 m/s^2
B.) 2.1 m/s^2
C.) 1.7 m/s^2
D.) 1.5 m/s^2
E.) 2.3 m/s^2

Homework Equations

F = ma

The Attempt at a Solution

1. First
   
   m_{1} = 6 kg
   m_{2} = 4 kg
   m_{3} = 9 kg
   
   \theta = 30°
   
   a = a_{x} = a_{y}
2. Then, the sum of forces on the three masses
   
   \sum F_{x1} = T_{2}-m_{1}g\sin\theta = m_{1}a
   \sum F_{y1} = 0
   
   \sum F_{x2} = T_{1}-T_{2}-m_{2}g\sin\theta = m_{2}a
   \sum F_{y2} = 0
   
   \sum F_{x3} = 0
   \sum F_{y3} = T_{1}-m_{3}g = m_{3}a
3. Combine F_{x1}, F_{x2}, & F_{y3} and isolate a...
   
   a = \frac{2T_{1} - g (m_{1}\sin\theta + m_{2}\sin\theta + m_{3})}{(m_{1} + m_{2} + m_{3})}[*]Solve for T_{1}
   
   \sum F_{y3} = T_{1}-m_{3}g = m_{3}a
   \sum F_{y3} = T_{1}-m_{3}g = 0
   T_{1} = m_{3}g = (9 kg)(9.81 m/s^2) = 88.29 N[*]Plug T_{1} into a and solve
   
   a = 2.1 m/s^2
   Or, answer B.
   

Is this the correct solution and answer? Did I solve correctly for T_1? I don't think it's right because the tension should not equal weight of m_3 because technically the block IS accelerating downward at this instant, isn't it?

 

[WillemBouwer]

smashd, ja if the tension in the rope was the same as the weight no acceleration will take place: Very important note: Define your axis for the entire system, I see you take the x-axis for your first 2 masses as parallel with the surface so the y-axis should be perpendicular to this surface for the entire system you cannot change the axis for the 3rd mass... After defining your axis, draw the 3 FBD's for the masses, you are on more or less the right track, let's see if we can get to solution here... Do the FBD first...

 

[smashd]

Thanks for the input, WillemBouwer.

So \sum F_{3y} should be:

\sum F_{3y} = m_{3}g - T_{1} = m_{3}a

Then a would become after combining the forces on the system:

\frac{m_{3}g - m_{2}g\sin\theta - m_{1}g\sin\theta}{m_{1} + m_{2} + m_{3}}Which is still 2.1 m/s^2, but this is the proper solution to the problem?

 

[WillemBouwer]

yes, that looks better, ja as you take the acceleration as positive downward the weight force should be positive aswell... and it can't be 0 as you stated...