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 11-11-2004, 08:10 AM #1 Mathiasdm LQ Newbie   Registered: Aug 2004 Distribution: Ubuntu Gutsy Gibbon Posts: 25 Rep: And some maths :-P Well, I'm sorry to bother you all with this exercise, but I can't seem to find the solution. First of all: no, this is NOT a homework, it's just an exercise in our book, and I'm curious as to what the solution is. I've been trying to find it for over an hour straight, and I just can't figure it out. A man starts to walk in point P and walks around a circular lake (radius: 3.2 km) with a constant speed of 4.8 km/hour. The position of the man is R. How fast does the length of the cord PR change, when PÔR = 60°? Oh, and we saw this exercise with Derivatives, just mentioning ;-) Anyone care to try?
 11-11-2004, 08:56 AM #2 vharishankar Senior Member   Registered: Dec 2003 Posts: 3,161 Blog Entries: 4 Rep: Can you post a link to a diagram of this problem, if possible?
 11-11-2004, 09:08 AM #3 Andrew Benton Senior Member   Registered: Aug 2003 Location: Birkenhead/Britain Distribution: Linux From Scratch Posts: 2,073 Rep: Or you could tell us where Ô is. The angle PÔR is meaningless if you don't define Ô. If Ô is the centre of the lake the length of the cord PR changes at 4.8*sin60=4.157km/hour when PÔR = 60° Last edited by Andrew Benton; 11-11-2004 at 09:09 AM.
 11-11-2004, 09:27 AM #4 Mathiasdm LQ Newbie   Registered: Aug 2004 Distribution: Ubuntu Gutsy Gibbon Posts: 25 Original Poster Rep: Oh, oops. Yes, Ô is the centre of the lake. Sorry! Could you explain your answer too?
 11-11-2004, 09:43 AM #5 kees-jan Member   Registered: Sep 2004 Distribution: Debian, Ubuntu, BeatrIX, OpenWRT Posts: 273 Rep: Explaining the answer would either involve a bunch of formulas (if you want calculus and derivatives used in the explanation) or a bunch of drawings (if you want an explanation using vector analysis), both of which are most easily provided by sitting at a desk with you. Maybe you can make our lives a little easier by explaining what you are trying to do and how far you've got? Thanks! Groetjes, Kees-Jan
 11-11-2004, 09:57 AM #6 Mathiasdm LQ Newbie   Registered: Aug 2004 Distribution: Ubuntu Gutsy Gibbon Posts: 25 Original Poster Rep: Well, I'm trying to calculate it using calculus and derivatives. I'm sorry for bothering you all with this, and I feel rather stupid asking this question (normally, I don't have any problems calculating derivatives). Here's a picture: http://img.photobucket.com/albums/v1...ol/picture.jpg
 11-11-2004, 10:16 AM #7 kees-jan Member   Registered: Sep 2004 Distribution: Debian, Ubuntu, BeatrIX, OpenWRT Posts: 273 Rep: Well, then. The length of a can be found using the cosine rule (using the angle POR). POR itself is a function of radus r, velocity v and time t. You can then compute the derivative of a using the chain rule (twice, I think). Where exactly do you get stuck? Groetjes, Kees-Jan PS. You are pushing "general" to its limit, aren't you :-)
 11-11-2004, 03:48 PM #8 Mathiasdm LQ Newbie   Registered: Aug 2004 Distribution: Ubuntu Gutsy Gibbon Posts: 25 Original Poster Rep: Silly me! I didn't think about the cosine rule ops: I understand it now :-) Thanks! PS. Yes, I know :-P

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