Physics science fair project. Need help fastt!!!?

So tomorrow I have my science fair project presentation and my project is a roller coaster made out of materials I found in my house that were reliable to make an appropriate project. My "roller coaster" consisted of a 4 feet high starting point and a loop that was adjustable to any radius ranging from 5... show more So tomorrow I have my science fair project presentation and my project is a roller coaster made out of materials I found in my house that were reliable to make an appropriate project. My "roller coaster" consisted of a 4 feet high starting point and a loop that was adjustable to any radius ranging from 5 inches to 6.5 inches. My question is "how does the radius of a circular loop affect the speed required for an object(marble) to complete the circular loop?" I tested 4 radius that were 6.5 in, 6 in, 5.5 in, and 5 in. I found the heights that each individual radius require the object to have in order to complete the loop but I'm still not sure how to conclude how the speed is affected by the radius of the loop. Can I get some helpp please!

My results were:
Radius 6.5 in = height of 1.8 ft
Radius 6 = height of 1.6 ft
Radius 5.5 = height of 1.4 ft
Radius 5 in = height if 1.25 ft
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