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# Relative Velocity Question watch

1. A few days ago I did an Add Maths Paper from CIE (you may have seen my thread on it) and in it there was this relative velocity question. Now owing to the fact that r. velocity was the topic that we covered in the least detail, and that too about three days before the exam, I wasn't too confident about it. The question ran like this:

A boy cycles south at 7m/s. A wind of constant speed 12m/s appears to be blowing from the north east. Find the actual direction of the wind.

I would really appreciate it if someone could work out the answer for this, I thought I might have got it right but I'm not so confident after thinking about it a bit.
2. A boy cycles south at 7m/s. A wind of constant speed 12m/s appears to be blowing from the north east. Find the actual direction of the wind.

call the wind's vector a
the boy's vector is -7j

As the boy is travelling at this vector, everything else appears to have a relative vector of +7j

So if the boy sees the wind travel at 12 south west, which is (12/√2)-i-j

Take +7 from that to find it's actual vector:

(12/√2)-i + (12/√2)-j - 7j = (12/√2)-i + (6√2 - 7)-j

I think... Haven't done this yet...
3. Hmm thx for trying, but I wouldn't have thought this one required i's and j's. I thought about it a bit and I got the diagram at the bottom. From there I used simple sin rule:

sin thetha/7=sin135/12

sinthetha=7*sin135/12

thetha=24.36 degrees

Therefore the actual angle of the wind is 360-135-24.36 degrees (for working out the bearing) which is 200.4 degrees.

Can anyone confirm if the diagram and working out is right?
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Updated: June 4, 2004
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