Luckily there are web resources
BE SURE to click "show steps"
You can change the question to .
That will help you find the second derivative or
The position vector of a point is given by . Prove that:
b) the angle between the position vector and the acceleration vector is constant. Calculate this angle.
Answer: b) pi/2
I tried to derive the position vector two times but it isn't working.
For the second question you can calculate the scalar product of the vectors like:
The scalar produt will tell you more about the angle between the vectors, but first you've to determine the coordinates of the vector r(t) (see Plato's post).