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jrcdude
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 Posted: Tue May 24, 2011 3:35 pm Post subject: 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
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| Posted: Tue May 24, 2011 3:50 pm Post subject: RE:Vectors Question |
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| The wind is blowing FROM bearing 118 degrees. |
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jrcdude
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| Posted: Tue May 24, 2011 8:23 pm Post subject: Re: Vectors Question |
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| Thank you, but this is known. |
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Brightguy
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| Posted: Tue May 24, 2011 10:33 pm Post subject: 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
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| Posted: Tue May 24, 2011 11:25 pm Post subject: 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. |
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