Radius of a projectile at the top of its path.

[CornMuffin]

Homework Statement

A projectile is fired at spead v₀ and angle $$\Theta$$. What is the radius of curvature of the parabolic motion
a) at the top?
b) At what angle should the projectile be fired so that the radius of curvature at the top equals half the maximum height?

(the projectile is fired on a horizontal plane)

Homework Equations

Radius of curvature is defined to be the radius of the circle that matches up with the path locally at a given point.

The Attempt at a Solution

I just need help getting started with this problem I'm sure I can finish it on my own, I'm just not sure how to find the radius of curvature on a parametric curve. I probably can do part b on my own after I figure out how to do part a.

 

[PhanthomJay]

What's the centripetal acceleration at the top? (Hint: There is no vertical speed at the top).