# Vectors Question

1. May 9, 2010

### AquaAqua

Hey, im new to the forum, but i've seen peoples help here and some of it is really good, but i have a question that i just dont get....

"A person travelling eastward at the rate if 4.0kmhr-1 observes that the wind seems to blow directly from the north after doubling his speed the wind appears to come from the north east, determine the direction of the wind and its velocity."

I tried drawing it on a graph but i got lost , i dont know what to do, and im sure its really easy to....
any help?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. May 9, 2010

### tiny-tim

Welcome to PF!

HeyHey AquaAqua! Welcome to PF!
As you've probably guessed, you need two vector triangles to solve this …

one has sides of lengths w (for the wind) and 4.0, and you know the directions of two of the sides …

the other has sides of lengths w (for the wind) and 8.0, and again you know the directions of two of the sides …

and you can simplify matters by putting the two triangles together, since the "wind" side is the same direction and length for each.

3. May 9, 2010

### AquaAqua

Re: Welcome to PF!

Hey tiny tim, thanks for your help, but im a bit physics retarded and i'm still not completley getting it.....

attached an image of what i think i need to do?
can you maybe clarify?
BIG THANKYOU

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4. May 9, 2010

### tiny-tim

Hey AquaAqua!

First, where did 12 come from? The two speeds east are 4 and 8.

Second, if w is meant to be the unknown speed Vwg of the wind relative to the ground (the same in both cases), then it isn't vertical (North).

Try again!

5. May 9, 2010

### AquaAqua

Hey Tiny-Tim

,because it implies his journey at now8kmhr-1 from the point he was at (whiich was 4km from the origin...

So is it a 3d diagram?

6. May 9, 2010

### tiny-tim

Hey AquaAqua!
ah, no, the distances don't matter at all in a question like this.

This question is about relative velocities, and all you need is the velocities (ie, their magnitudes and directions) themselves.

The question says "after doubling his speed", which as you say is 8 kmhr-1.
No, it's two 2D diagrams stuck together … still 2D!

Your original diagram was the right idea, you've just labelled it wrong.

Redraw it, with w (the velocity of the wind relative to the ground) not from the north or north-east.