Question

You are riding in a boat whose speed relative to the water is 7.2 m/s. The boat points at an angle of 25.7° upstream on a river flowing at 14.1 m/s. Find the time it takes for the boat to reach the opposite shore if the river is 24.5 m wide.

Answer 1

If we consider the direction across the river as 0deg, then the resultant velocity of the boat is

(angle a is 25.7 deg)

(7.2 cos a, 7.2 sin a) + (0,14.1 sin -90)
= (6.386,3.122) + (0,-14.1)
= (6.386, -10.978)

which is 10.835 in the direction -59.49 deg

So, now we want a course in the same direction, where the x-distance is 24.5

If d is the distance traveled, then 24.5/d = cos -59.45
d = 48.25

48.25/10.835 = 4.45 sec

Answer 2

I understand the method you used completely but when I went to enter in the answer it incorrect...any help?