# Newton's 2nd law problem

1. Nov 19, 2012

### Ammar w

1. The problem statement, all variables and given/known data

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

2. Relevant equations

Ʃ$\vec{F}$ = m$\vec{a}$

3. 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.

Last edited by a moderator: May 6, 2017
2. Nov 19, 2012

### voko

What is direction of T for 2M?

3. Nov 19, 2012

### aralbrec

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