Question

Jeffrey wants to swim his girlfriend who is straight across from him, on the other side of a river bank. The river has a current velocity of 4km/h [W]. The width of the river is 300m. If it takes Jeffrey 15 minutes to reach her,

a) how fast should the boy swim to cross the river in 15 minutes?
b) what is his speed with respect to the river, as seen by the girl's father coming after the two with a loaded gun in a boat?
c) What should Jeffrey's heading be in order for him to end up exactly where his girlfriend is?

For question a)I got 4.2 km/h, and for c) I got a heading of 16.7 degrees.

I don't understand how to obtain the answer for b). Please help

Answer 1

d = 300 m = 0.3km
t = 15 min. = 0.25 h.

a. V = d/t = 0.3km/0.25h = 1.2 km/h

c. Tan A = 4/1.2 = 3.333
A = 73.3o West of North due to wind.

Heading = 73.3o East of North = 16.7o
CCW.



Vw + Vs = 1.2i
-4 + Vs = 1.2i
Vs = 4 + 1.2i = 4.18km/h[16.7o] CCW = 4.18km/h[76.3o] East of North.