Figure 4.31
(a)
Four forces act on a supertanker:
and
are the tension forces due to the towing cables,
is the forward drive force produced by the tanker's engines, and
is the force with which the water opposes the tanker's motion.
(b)
The free-body diagram for the tanker.
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and 
are the tension forces due to the towing cables, 
is the forward drive force produced by the tanker's engines, and 
is the force with which the water opposes the tanker's motion.