1. The problem statement, all variables and given/known data In the picture: http://postimg.org/image/6gdkm5vpx/ 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 ?