Determining Speed of Toy at Top of Loop-the-Loop

[dranseth]

Homework Statement

A toy with a mass of 100g is going through a loop-the-loop with a diameter of .5m. How fast must the car be moving at the top of the loop in order to not fall.

The Attempt at a Solution

At the top of the loop, there is a normal force (Fn) and the force of gravity (Fg). I'm not sure how to do this. Do I neglect one of the forces? Set one to zero, or what?

 

[Doc Al]

What happens to the normal force when the car is just about to lose contact with the track?

 

[dranseth]

Fn approaches 0. Thanks.