1. Find the total range. (lets call it d )
2. find the horizontal distance covered by it in time t = 4. (call it x)
4. the radius will be the min distance of item at t=4 from d/2.
5. distance from d/2 wil be d/2 - x.
6. Draw the right angle triangle joining item, d/2 point
7. find out the vertical height of the item at t = 4.
8. Now draw the right angle triangle joining d/2, height, and radius.
9. In this triangle you have the angle, 2 lengths. Find the length of hypotenuse of this triangle, which is going to be the radius of curvature.