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I am having trouble understanding the reasoning behind the solution in this Irodov General Physics problem:

problem 1.6.) A ship moves along the equator to the east with velocity $v_0=30\;km/h$. The southeastern wind blows at an angle $\theta=60°$ to the equator with velocity $v=15\;km/h$. Find the wind velocity $v'$ relative to the ship and the angle $\theta'$ between the equator and the wind direction in the reference frame fixed to the ship.

My calculations:

My diagram

After drawing the initial diagram I consider that in order for the wind direction to be relative to the boat, I need to subtract the boat vector from the wind vector.

In my diagram I have repositioned the boat vector to be subtracted from the wind vector and using trigonometry I solve as follows:

First the cosine law to determine the magnitude

The magnitude = $\sqrt{30^2 + 15^2 - 2\cdot 15\cdot 30\cdot \cos{60}}$

= $\sqrt{675} \approx 25.98$

Then for the angle I can use sine law

$\frac{sin(\theta)}{30\frac {km}{h}} = \frac{sin(60)}{\sqrt{675}}$

$\theta= \arcsin{\frac{30\frac {km}{h}\cdot sin(60)}{\sqrt{675}}}$

$\theta = 90°$

The answer I get is therefore that the wind, in reference to the boat (as felt by the people standing on the boat), is approx $26\:km/h$ at an angle of $60°$ South of West (in relation to the equator).

However the answer given in the solutions is always that the approximate speed of the wind is $40\:km/h$ and the angle is approx $19°$.

I have looked at the solutions and I think the best explanation is given in the following diagrams obtained from link #3:

Diagram

Formula

I do not understand, however, why he is using $\cos{(\pi +\theta)}$. I want to understand why my solution is incorrect and yields a different answer. What is my mistake and what is the proper intuition for understanding this problem if I have done so incorrectly?

Solutions I have viewed:

  1. This Youtube Video

  2. This online blog

  3. This online blog

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2 Answers 2

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A South Easterly wind comes from the South East your diagram seems to show a North Easterly wind. As the ship is steaming into the wind the apparent velocity should be greater than the true velocity.

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    $\begingroup$ Sorry this answer is so short, but I'm lying in bed using my iPad, I will flesh it out tomorrow morning. $\endgroup$
    – vascowhite
    Commented Jul 12, 2014 at 22:40
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    $\begingroup$ I am banging my head on the wall... I understand my mistake. I will give you the answer once you decide to write it--a more complete solution that is. $\endgroup$
    – Klik
    Commented Jul 12, 2014 at 22:48
  • $\begingroup$ The calculations all work out according to the solution, but, just a thought, why do the diagrams seem to indicate the wind is blowing in the South Eastern direction as well? $\endgroup$
    – Klik
    Commented Jul 13, 2014 at 18:26
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I think you are referring to the I.E.IRODOV (Problems in General Physics) question number 1.6 (KINEMATICS)

South-Eastern winds don't go towards South-East, in fact it's the complete opposite, they originate from South-East and go towards North-West.

So, if you were to reverse the direction of the wind vector, the solution would be correct and so will be the answer obtained by you.

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