Calculating Speed in Circular Motion: A Roller Coaster Conundrum

[fightboy]

A 400kg roller coaster cart travels along the inside of a vertical circular track of radius 10 m. At its highest point it moves at 20 m/s. Friction between the cart and track is negligible.
Find the cart's speed one quarter of a loop later (when it's at the same height as the loop's center).
I know there is radial acceleration due to the change of the velocity vector around the circular path, but how do I know the speed is changing? Also how can I determine what the speed will be at that exact point along the circular path? Any help is appreciated! I'm really confused on how to set up this problem

 

[Simon Bridge]

The tools you can bring to bear are:
1. your understanding of circular motion
2. a free-body diagram
3. conservation of energy

 

[fightboy]

ah ok i forget to apply the conservation of energy equations to this problem, I'll give it a go thanks!