Roller coaster physics conceptual question

[Ryan Saunders]

Homework Statement

For my high school physics class we made roller coasters out of card stock with various loops and funnels. We then had multiple questions to answer. I couldn't seem to figure this one out.

Why does the power have to be found at the bottom of the loop? Explain in terms of both energy and velocity.

Homework Equations

Power=Work/time
Work=Force*Distance or the change in kinetic energy
Force=Mass*Acceleration

The Attempt at a Solution

I figured that the conversion from kinetic energy (at the bottom of the loop) to potential energy (at the top of the loop) could have an affect on the power. But I was not sure about how velocity played a part. I also know that gravity is constantly pulling down on the marble and force normal is keep it on the track, but like before I didn't know how to incorporate it.

 

[CWatters]

I think we need to see earlier parts of the question to understand the context.