rollercoaster centripetal force

[kfcman289]

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what is supplying the centripetal force on a rollercoaster that goes upside down in a loop? how does the force change throughout the circle?

 

[DrknoSDN]

Gravity and the normal force.
At the bottom the centripetal force is provided by both gravity and the normal force while at the top it is the normal force minus gravity.

 

[kfcman289]

Gravity and the normal force.
At the bottom the centripetal force is provided by both gravity and the normal force while at the top it is the normal force minus gravity.

But shouldn't the normal force only equal a small component of gravity? Wouldn't the normal force always be cancelled out by something else and would never be able to supply a centripetal force?