Exercício de Dinamica Aplicada (34)

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195 
 
 
PROBLEM 11.131 
When a small boat travels north at 5 km/h, a flag mounted on its 
stern forms an angle 50θ = ° with the centerline of the boat as 
shown. A short time later, when the boat travels east at 20 km/h, 
angle θ is again 50°. Determine the speed and the direction of the 
wind. 
 
SOLUTION 
We have /W B W B= +v v v 
Using this equation, the two cases are then graphically represented as shown. 
 
With Wv now defined, the above diagram is redrawn for the two cases for clarity. 
 
Noting that 
 
180 (50 90 )
40
θ α
α
= ° − ° + ° +
= ° −
 
180 (50 )
130
φ α
α
= ° − ° +
= ° −
 
We have 
5 20
sin 50 sin (40 ) sin 50 sin (130 )
W Wv v
α α
= =
° ° − ° ° −