Homework Help: Easy newtons law problem

1. The problem statement, all variables and given/known data
Check out my attachment to get an idea what is happening. What we have is an inclined plane with 2 masses (m1 and m2) which are touching one another and the angle of the incline is 30 degrees.
The coefficient of kinetic friction between m1 and the plane is 0.23 = mu_k1
The coefficient of kinetic friction between m2 and the plane is 0.51 = mu_k2
m1 = 2.5kg
m2 = 3.1kg
theta = 30 degrees
The question is to find the acceleration of the pair of masses, and the second question is find the force of m1 on m2

2. Relevant equations
F=ma


3. The attempt at a solution
I applied Mr Newton on each mass individually to get
for mass m1:
x direction (parallel to the inclined plane):
[tex]-F_{21}-\mu_{k1}N_{1}+m_{1}g\sin(30)=m_{1}a[/tex]
y direction (perpendicular to the inclined plane):
[tex]-m_{1}g\cos(30)+N_{1}=0[/tex]

for mass m2:
x direction:
[tex]F_{12}-\mu_{k2}N_{2}+m_{2}g\sin(30)=m_{2}a[/tex]
y direction:
[tex]-m_{2}g\cos(30)+N_{2}=0[/tex]

I used [tex]F_{12}=-F_{21}[/tex].

I put all that into the mix and out popped a = 9.33m/s^2, which is wrong.
The answer says a = 1.6m/s^2. I am pretty sure my maths is ok. I'd say the problem is in my application of Newton but can't see anything wrong.

Thanks in advance

 

thanks alphysicist you have solved the problem