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# vector direction cosines watch

1. Can someone help me find where I've gone wrong? (Q2) I got a different answer to the notes
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2. if 5a/4 = 5b/-13 = c

let c = 1 without loss of generality

5a/4 = 1

5b/-13 = 1

a = 4/5

b = -13/5

c = 1

these are the three components of the vector n

the direction cosines can be read off from the unit vector .... the magnitude of n is (√210)/5

so the first direction cosine is ( 4/5 ) / (√210)/5

etc
3. (Original post by pippabethan)
Can someone help me find where I've gone wrong? (Q2) I got a different answer to the notes
I get by taking the cross product.

[And having thought about the OP's working, I think that the cross product approach is quicker]
4. (Original post by atsruser)
I get by taking the cross product.

[And having thought about the OP's working, I think that the cross product approach is quicker]
i was not sure if they had to know about the x product
5. (Original post by pippabethan)
Can someone help me find where I've gone wrong? (Q2) I got a different answer to the notes
As you can see from the bear's working, the components of your vector are (a,b,c) - you are:

a) working with components (5a/4, -13b/5, c)
b) failing to divide by the length of (a,b,c)
6. (Original post by the bear)
i was not sure if they had to know about the x product
They do. See top of the picture. My Mycroft Holmes-like power of observation saves the day yet again. They are also using the old fashioned wedge notation, so she is probably at somewhere with time mellowed wood panelling on the walls. They tend to go together.
7. Thank you everyone!! I've now corrected it, and done it the cross product way (much quicker, but didn't think of it haha)
8. (Original post by pippabethan)
Thank you everyone!! I've now corrected it, and done it the cross product way (much quicker, but didn't think of it haha)
It never hurts to be aware of multiple methods though. Is this in fact A level stuff? Which syllabus has direction cosines?
9. (Original post by atsruser)
It never hurts to be aware of multiple methods though. Is this in fact A level stuff? Which syllabus has direction cosines?
Technically I'm at uni, but I'm doing a science course, and I figured the stuff would probably be simple enough to be in fp modules!

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Updated: January 13, 2017
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