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Question

A seagull flies at a velocity of 9.00 m/s straight into the wind. (a) If it takes the bird 20.0 min to travel 6.00 km relative to the Earth, what is the velocity of the wind? (b) If the bird turns around and flies with the wind, how long will he take to return 6.00 km? (c) Discuss how the wind affects the total round-trip time compared to what it would be with no wind.

a) $-4.00 \textrm{ m/s}$

b) $462 \textrm{ s}$

c) see video for derivation

Solution Video

# OpenStax College Physics Solution, Chapter 3, Problem 53 (Problems & Exercises) (13:26) Rating

6 votes with an average rating of 4.

## Calculator Screenshots Video Transcript

Submitted by blueFZ07 on Sun, 10/03/2021 - 19:14

Why not Vsa = Vsg + Vga like the relationships you did in 52? I'm not understanding how to know what the given V relates to and then how to set up the equation the right way. Please explain. Thx.

Submitted by blueFZ07 on Sun, 10/03/2021 - 19:54

I don't understand the setup around 2:10. Vag points left - shouldn't it be negative? and then at 3:40, why do you turn Vsa around to point left?

Submitted by ShaunDychko on Mon, 10/04/2021 - 10:58

Hi blue, thanks again for the questions. At 3:25 $v_\textrm{ag}$ turns out to be negative, as you predicted. I think what you were expecting is to see a minus before the $v_\textrm{ag}$ term, but the negative is instead in the value of $v_\textrm{ag}$. The formula has only positives since it says "combine these velocities together" - which means add them. The $v_\textrm{ag}$ turns out to be negative, so adding $v_\textrm{ag}$ to $v_\textrm{sa}$ reduces $v_\textrm{sa}$. We have ended up adding a negative... yeah, I can see why this is confusing, but I hope this is helping.
Occasionally you will see some formulas with minus signs in them in solutions for other types of problems, in which case the variables are just magnitudes (always positive in other words), but that's not the case for this type of problem where the negatives have to be treated carefully.
At 3:40 I'm drawing $-v_\textrm{sa}$. The negative means draw $v_\textrm{sa}$ in the opposite direction.