Figure 4.32
(a)
The force
acts on the truck and propels it forward. The drawbar exerts the tension force
on the truck and the tension force
on the trailer.
(b)
The free-body diagrams for the trailer and the truck, ignoring the vertical forces.
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acts on the truck and propels it forward. The drawbar exerts the tension force 
on the truck and the tension force 
on the trailer.