# Homework Help: Relative velocity question from giancoli physics

1. Aug 18, 2011

### hamsterpower7

1. The problem statement, all variables and given/known data
I attached the picture on the bottom (correction on the picture 30 degrees ---> 35 degrees sorry about that)
Vpa: 600 km/h
Vag:100 km/h

and the question is to find the angle theta
2. Relevant equations

3. The attempt at a solution

So I tried using Pythagorean theory by setting x and y

$$(V_pa)^2 =y^2 + x^2$$

set $$y = x/ (tan 35)$$

so then I solved x = 344.1458617 Km/h

and since $$Sin/theta = (V_ag + x)/V_pa$$

$$/theta = 47.75271609$$

sorry for ignoring the scientific notation,
I am not used to it :(

um anyways my answer was wrong
because the "solution" chose to use the 'sine law'
and that gives the right answer but I wanted to know what I did wrong to get the wrong answer

much appreciated

Last edited: Oct 2, 2011
2. Aug 18, 2011

### PeterO

Your diagram shows right angled triangles. The real situation may not be right angled, which would necessitate use of sin rule or cos rule

3. Aug 18, 2011

### hamsterpower7

Okay I will post the question up to see if it wasn't a right angle

An airplane, whose air speed is 600km/h, is supposed to fly in a straight path 35 degrees north of east
but a steady 100km/h wind is blowing from the north
in what direction shuold the plane head?

4. Aug 18, 2011

### PeterO

$$(V_pa)^2 =y^2 + x^2$$
$$(V_pg)^2 =y^2 + x^2$$
and since you don't know $$(V_pg)$$ it doesn't get you anywhere.