Problem understanding Newtons third law ...

[Warlic]

Homework Statement

[/B]
Three blocks move on a frictionless surface and a 42N force acts on the 3kg block.
The force exerted by the 1 kg block on the 2 kg block is 14N.

What is then the force exerted by the 2 kg block on the 1kg block? If it is 14N, then shouldn't the 2kg block stay in place and not move? Shouldn't then the 1kg block also stay in place?

2. Homework Equations
F=ma?

The Attempt at a Solution

 

[Warlic]

This would be the correct figure; 3kg block pulled by a force of 42N, and the 1 kg and 3kg block are connected by a string.

 

[stockzahn]

If it is 14N, then shouldn't the 2kg block stay in place and not move? Shouldn't then the 1kg block also stay in place?

Don't forget the force applied due to the string connecting the blocks; the 14N-force isn't the only one affecting the block 1/2 - system.