# Boat velocity homework

1. Feb 7, 2005

### Spyder1121

I've been working on this problem for 3 hours atleast now. I just need to know how to work it. The problem has a boat moving southeast at 0.380 m/s relative to the earth, and the river is flowing east at 0.460 m/s relative to the earth. I need to know how to get the veloctiy of the boat relative to the river. Any help is much appreciated!

2. Feb 7, 2005

### Spyder1121

i know that, but that answer would not work on 'mastering physics'

3. Feb 7, 2005

### dextercioby

Being given (or having obtained) the components along mutually ortonormal axis,u can compute the modulus by using Pythgora's theorem...

I think the (stupid) computer wouldn't mind...I hope,for your sake...

Daniel.

4. Feb 7, 2005

### Spyder1121

i know. but here's what I've done.. V of the canoe to the river = V of the canoe to the earth - V of the earth to the river. so using my numbers I have : V= 0.380-0.460 and I get an answer of -0.08, but the question just wants the magnitude so i would use 0.08, but that doesn't work. can you tell me what I'm doing wrong?

5. Feb 7, 2005

### Spyder1121

i've tried that as well. I have the second part of the question. It asks for me to find the direction of the velocity of the canoe relative to the river.
and that answer is 54.6 degrees

6. Feb 7, 2005

### Spyder1121

yes i've done that as well and I get the answer to be .266 m/s correct? This answer did not work as well.

7. Feb 7, 2005

### Spyder1121

but when I do the arctan of .38/.46 i don't get 54.6 which I know is the correct answer for part 2

8. Feb 7, 2005

### Spyder1121

okay i have all of the numbers.... now what do I do with them to find the V of the boat in respect to the river??

9. Feb 7, 2005

### Staff: Mentor

$$\vec{V}_\textrm{(boat/earth)} = \vec{V}_\textrm{(boat/river)} + \vec{V}_\textrm{(river/earth)}$$
Solve for x & y (east and north) components separately.