Finding the acceleration of an object

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

In the picture:

http://ift.tt/NgW0t4

There is a force (F) pushing the car to the right

The car's mass is M

There are two other objects with masses m1 and m2 that are connected by a cord with no mass.

There is no friction

In terms of m1, m2 and M find the acceleration needed to keep m1 and m2 from falling down.



2. Relevant equations



This is the second Newton's law.

The only equation I use is



Fnet=m*a



3. The attempt at a solution



I have tried doing this by placing the 3 m's (m1, m2 and M) in their respective Cartesian planes to get the forces of each one, but the part I don't quite understand is which forces I have to put in the formula...

I understand that I need to add the masses:

Fnet=(M+m1+m2)*a

and I understand that Fnet consists of the F force that is affecting the car, but what other?

Do I need to use the tension of the cord that holds m1 and m2 together? or all I do is:

F=(M+m1+m2)*a

So the acceleration would be

F/(M+m1+m2)=a ?