Why is the 4kg carriage's kinetic energy zero in this problem?

[pyroknife]

I attached the problem and the solution.

The book uses V has potential energy and T as KE.

I'm not quite sure how they did this problem. I was following it until the 2nd to last line of equations. where they have "8.829-11.772+1/2*6*v^2+0=0"
Why don't they have the KE for the 4kg carriage? It looks like they said that was 0, but isn't that still moving too?

 

[gneill]

I attached the problem and the solution.

The book uses V has potential energy and T as KE.

I'm not quite sure how they did this problem. I was following it until the 2nd to last line of equations. where they have "8.829-11.772+1/2*6*v^2+0=0"
Why don't they have the KE for the 4kg carriage? It looks like they said that was 0, but isn't that still moving too?

What freedom does it have to move when (at the instant) the links are inline?