Question (based off 2007 VCAA Exam 2)

[hyperblade01]

I can do 4a), it's just there to show what r(t) is...

My question is regarding d):
The solutions say to use dot product of r and v to find when it is closest to the base. I don't understand why    Sorry if i'm a bit vague...



Other question is: Easy, average or hard exam? I got slightly less than what I got in the 06 exam due to careless mistakes but apart from that I found it easy-average (minus 4d) :p

[TonyHem]

4D - I asked this question too : http://vcenotes.com/forum/index.php/topic,15576.15.html
Comparison: I'd say average. I also did better on 06 :/

[enwiabe]

Well, that was my exam in '07. On the day, I found it to be a very reasonable exam. I got bogged down on the forces question because I had a negative in the wrong place and I was coming up with a completely illogical answer and couldn't work out why until afterwards.

As for that question, I've drawn it in paint for you. The position vector maps to different points on the flight's trajectory. This position vector CHANGES direction as designated by r1, r2, r3 etc. at different times. The velocity vector, however, has the same gradient throughout because it's linear and so will always point in the same direction. So when the position vector is perpendicular to the velocity vector, you'll get the minimum distance. Anywhere else and you can make a right-angled triangle out of it with the other position vector not at right angles with the position vector being the hypotenuse (therefore being at a larger distance away).

Hope this was of help

[hyperblade01]

I can definitely see why you can do r.v now , thank you very much!