Science Buddies: "Ask an Expert"

I am assisting my daughter with the experiement "how much height to loop the loop." We used a constant 15-in diameter for loop for all trials. We used 3 different Rise heights: 43 in, 40 in, & 36 in. and 2 different runs: 20 in & 24 in. All 10 test runs for both the 43 & 40 in rises were successful at 20 & 24 in runs. Test runs at 36 rise at 20 in run did NOT work-marble fell out half way around loop. We increased run to 24 in, then all 10 test runs successfully made it through loop. Why did this work by just changing the run? We need to explain why.

Jillychris - The attribute of the marble that allows it to successfully enter the loop and stay on the track at the highest point of travel in entry velocity. Centripetal force must be greater than the force of gravity at that maximum point height. This is a function of velocity of the marble. I would appear that the extra 4" of travel allowed the marble, from a initial velocity of zero, to accelerate an additional amount such that you achieved the minimum velocity required to stay on track successfully. The marble loses velocity as it rolls upward to the highest point, then starts to accelerate again during it's downward path in the loop. Even though the potential energy was the same (descent from 36 inches), that extra 4 inches of accelerating travel raised the entry velocity. Good experiment, you successfully determined the near exact point of failure. To continue, you might refine that experiment and do the test at 21", 22" 23" and other points in between to see where you consistently get positive results.

Rick Marz