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jrcdude

Tue May 24, 2011 3:35 pm


Vectors Question

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So this was a question on my recent vectors test:

Ship is sailing at a heading of 173 degrees at a speed of 35 knots.  Wind is blowing from a bearing of 118 degrees at a speed of 16 knots.  Determine the resultant speed.

We were asked to use cartesian vectors to solve. (Correct? answer should be 29 knots)  Using this method, I get a different answer then if I did cosine law (46 knots).

Can someone show me what is going on with this, and which is truly the correct answer?


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DemonWasp

Tue May 24, 2011 3:50 pm


RE:Vectors Question

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The wind is blowing FROM bearing 118 degrees.


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jrcdude

Tue May 24, 2011 8:23 pm


Re: Vectors Question

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Thank you, but this is known.


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Brightguy

Tue May 24, 2011 10:33 pm


Re: Vectors Question

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Since the ship is travelling INTO the wind, the speed should decrease, not increase.

You probably used the wrong angle in the cosine law: using the angle between the two vectors will give you the magnitude of their difference, not sum.


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jrcdude

Tue May 24, 2011 11:25 pm


RE:Vectors Question

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Ah I think that was the problem.. Should have noticed they were tail to tail so I should have used cos(180-x).  Thanks for your help.