Calculating Minimum Speed at Top of Vertical Loop

[SamLing2000]

Homework Statement

A 330 kg roller coaster car sits on a horizontal track. Ahead of it is a vertical loop with radius of 6.85 m. The car is given an initial speed of 23.4 m/s and the car successfully traverses the loop. What is the maximum amount of energy taken away from the car by friction so that the car successfully travels through the loop? (Hint: think about the normal force that the track exerts on the car at the top of the loop, this should give you a minimum speed at the top of the loop.)

m=330
vo = 23.4
r=6.85

Homework Equations

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The Attempt at a Solution

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I am sorry but I have no clue as to how to approach this problem. Please point me in the right direction, hints and suggestions are extremely welcome.
I also find a Hint for the problem but i wasn't able to make as much use of this one as I thought.
(Hint: think about the normal force that the track exerts on the car at the top of the loop, this should give you a minimum speed at the top of the loop

 

[SamLing2000]

Any help is appreicated.

 

[azizlwl]

Have you learned about work-energy theorem?