# Homework Help: Related rates

1. Oct 23, 2007

### bondgirl007

1. The problem statement, all variables and given/known data

Ship A travels at 10 knots on a due north course and passes a buoy at 8am. Ship B, travelling on a due east course passes the same buoy at 10am. How fast are the ships separating at 12 am?

2. Relevant equations

3. The attempt at a solution

I'm not familiar with knots and don't know what knots are in km/h. Also, how fast is Ship B traveling then?

2. Oct 23, 2007

### HallsofIvy

Why would you want to change to km/h? The problem doesn't require that you do. A "knot" is, as the problem implies, a unit of speed (it is one nautical mile per hour but you don't need to know that either to do the problem- just give the answer in knots).

However, in your second question you hit on the difficulty. You can't possibly answer this without know how fast ship B is traveling and apparently that is not given.

3. Oct 23, 2007

### bondgirl007

Would it be possible to find the speed of B by comparing the time it takes for both A and B to pass the buoy?

4. Oct 23, 2007

### stewartcs

I don't see how you could. You know nothing of the distanced travelled (which is what you are really after in the first place).

5. Oct 23, 2007

### bondgirl007

The time is 4 hours so can't you multiply that by 10 knots to find the distance of A?

6. Oct 23, 2007

### stewartcs

Sure, but that won't tell you the speed of B. It will tell you the distance travelled by A which is one part of the problem. If you knew the speed of B, you could take the same approach and find the distance travelled by B (B's speed x 2 hours).

Then you could use that information to find the rate of change between the two by using the Pythagorean theorem (differentiating it of course).

Last edited: Oct 23, 2007