Scalar product of position vectors

[nokia8650]

http://img520.imageshack.us/img520/9580/56788025.th.jpg


See the problem above. I can do all of the problem, barring the last part. I have found r and r.r:

http://img520.imageshack.us/img520/1590/29349935.jpg

How does this allow me to find the minimum and maximum distance?

Thanks in advance

 

[CompuChip]

Suppose, in general, that you have a function f(t). How do you find its minimum and maximum (i.e. the extrema)?

[Hint: it involves differentiation]

 

[nokia8650]

find values of t for f'(t) =0

I don't understand the significance of r.r, however.

Thanks

 

[CompuChip]

Well, r is the vector that describes the difference in position.
r . r is the square of its length. So the square of the distance between the particles.

Note that minimizing (maximizing) the distance is equivalent to minimizing (maximizing) the square of the distance.