# Homework Help: Question on vectors ?

1. Feb 28, 2008

### terainfizik

1. The problem statement, all variables and given/known data
A boat needs to sail due north , but the current is flowing southeast at a speed of 20km/hr. In still wate, the boat has a maximum speed of25km/hr .

(a) In what direction will the boat actually move (relative to an observer on land)?

(b) What's the course (direction) necessary to that ship could travel North ?

(c) What is the speed of the boat relative to an observer on land?

2. Relevant equations

N/A

3. The attempt at a solution

(a) North .

(b) East . I using vector theory .

(c) Using theorem phythogoras :

Speed^2 = 25^2 - 20^2
= 225
speed = 15 km/hr

2. Feb 28, 2008

### tiny-tim

No.

As you know, Pythagoras' theorem only works for right-angled triangles, and you don't have a right-angled triangle here.

You only know two directions and two lengths and the angle between them. That gives you a triangle for (a) and another one for (b).

Draw the triangles (only roughly - I'm not suggesting you measure them!), to make sure you've got them the right way round.

Then use sines or cosines to work out the unknown length and the unknown angle!

3. Feb 28, 2008

### HallsofIvy

Remember that the "sine law" and "cosine law" let you use sine and cosine on non-right triangles. As tiny-tim said, start by drawing a picture and identifying the parts of the triangle.