How Do You Incorporate Drag Force in Free Body Diagrams for Multiple Objects?

[MeesaWorldWide]

Homework Statement

Three toy ducks on wheels are attached to the same rope and a boy begins pulling one end of the rope with a force of 10 N. The first duck (duck 1) has a mass of 1.0 kg, the middle duck (duck 2) has a mass of 5.0 kg, and the third duck in line (duck 3) has a mass of 2.0 kg. I already calculated the acceleration of the three ducks in part a of this question (1.25 m/s^2). The next thing is to find the tension force in the rope connecting ducks 1 and 2. I don't understand how to go about doing this: any help is appreciated!

Relevant Equations

F = ma

I don't even know how to begin this. I know that I need to somehow account for the drag force that duck 3 is causing on the first 2, but I don't know how to deal with that. I am asking for someone to help me get started, not to give me the answer.

 

[haruspex]

There can be shortcuts, but the standard procedure is to draw a free body diagram for each duck and assign unknowns to the forces between them in a consistent manner.
Then write the F=ma equation for each duck.