# Having trouble with a relative motion problem

1. Feb 20, 2006

### frankfjf

Two ships, A and B, leave port at the same time. Ship A travels northwest at 20 knots and ship B travels at 27 knots in a direction 43° west of south. (1 knot = 1 nautical mile per hour; see Appendix D.) What are (a) the magnitude (in knots) and (b) direction (measured relative to east) of the velocity of ship A relative to B? (c) After how many hours will the ships be 150 nautical miles apart? (d) What will be the bearing of B (the direction of the position of B) relative to A at that time? (For your angles, takes east to be the positive x-direction, and north of east to be a positive angle. The angles are measured from -180 degrees to 180 degrees. Round your angles to the nearest degree.)

I am able to obtain the answers to a-c, but my textbook is not very helpful on how I'm supposed to find the answer to d.

I get 34.1 knots for a, 83 degrees for b, and 4.4 hours for c. I can't for the life of me figure out how to approach d. Please help!

2. Feb 20, 2006

### Integral

Staff Emeritus
Draw a picture, you need the vector between the final positions. Think about it, perhaps you are making it hardier then it is.

3. Feb 20, 2006

### frankfjf

I still don't get it. Are there any other hints you can give me?

4. Feb 20, 2006

### Integral

Staff Emeritus
You need to find the difference between the final positions of the ships.

5. Feb 20, 2006

### frankfjf

I'm a little confused as to how to get the final positions.

I have the following hunch: If 1 nautical mile per hour = 1 knot, and I know that t = 4.4, couldn't I just add 4.4 knots to each original magnitude and then go from there?