How Does the Angle Impact Force Calculation in Newton's Second Law Problems?

[Ammar w]

Homework Statement

http://www.diigo.com/item/image/2sb3i/ahcq

Homework Equations

Ʃ\vec{F} = m\vec{a}

The Attempt at a Solution

For M :
T - mg = ma
T - 19.6 = 2a (1)

For 2M :
40 cos (150) - T = 4a
from (1) : T = 2a + 19.6
substituting :
40 cos (150) - (2a +19.6) = 4a
40 cos (150) - 19.6 = 6a

a = \frac{40 cos (150) - 19.6}{6} = -9 m/s^2

but the right answer is 2.5 m/s^2.
if I replace 150 with 30 the answer will be correct
but it's 150 with respect to the positive x axis.
what is the wrong please??

 

[voko]

What is direction of T for 2M?

 

[aralbrec]

The sign of F cos 150 does not match what you've assumed in your equation.