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Narik
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QP: http://www.srepapmaxeeeerf.org/A%20L...%202006-01.pdf
MS: http://www.srepapmaxeeeerf.org/A%20L...06-01%20MS.pdf

I'm stuck on Question 4. What I did was to draw the force P, which is 7N and acting North. Then I drew the resultant force, which was 10N and acting 120* from North.

Then to find Q, I drew a line joining the tip of P to the tip of the resultant force. I used the cosine rule, but that gave me 8.9N, and the answer is 14.8N. The mark scheme uses vector notation with the 'i's and j's,' but why can't you do it the way I've done it?

Thanks.
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Farhan.Hanif93
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(Original post by Narik)
QP: http://www.srepapmaxeeeerf.org/A%20L...%202006-01.pdf
MS: http://www.srepapmaxeeeerf.org/A%20L...06-01%20MS.pdf

I'm stuck on Question 4. What I did was to draw the force P, which is 7N and acting North. Then I drew the resultant force, which was 10N and acting 120* from North.

Then to find Q, I drew a line joining the tip of P to the tip of the resultant force. I used the cosine rule, but that gave me 8.9N, and the answer is 14.8N. The mark scheme uses vector notation with the 'i's and j's,' but why can't you do it the way I've done it?

Thanks.
I think you've given us some but not enough information i.e. write out the steps of the cosine rule, it should be fine to use. You've prob just made a numerical error or something.
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olipal
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You can do it with the cosine rule What equation did you use?
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Noble.
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(Original post by Narik)
QP: http://www.srepapmaxeeeerf.org/A%20L...%202006-01.pdf
MS: http://www.srepapmaxeeeerf.org/A%20L...06-01%20MS.pdf

I'm stuck on Question 4. What I did was to draw the force P, which is 7N and acting North. Then I drew the resultant force, which was 10N and acting 120* from North.

Then to find Q, I drew a line joining the tip of P to the tip of the resultant force. I used the cosine rule, but that gave me 8.9N, and the answer is 14.8N. The mark scheme uses vector notation with the 'i's and j's,' but why can't you do it the way I've done it?

Thanks.
cos120 = -0.5

So Q = (149+70)^0.5 not (149-70)^0.5
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Narik
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(Original post by Noble.)
cos120 = -0.5

So Q = (149+70)^0.5 not (149-70)^0.5
Hey.
-------
Sorry guys, I ****ed up the cosine rule.
Thanks anyway.
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