Physics of zorbing

By Tue Nguyen

http://www.youtube.com/watch?v=tG3pgV8Crzk
Zorbing is the sport of rolling down a hill in an orb made of transparent plastic. The orb has 2 sections, one ball inside another with a layer of air in between to protect the rider by acting as shock absorber. The orb is made of flexible plastic so that protect the rider as well.
Assuming the ball roll down a hill of 20. m without slipping, I am interested in finding the final speed of the orb at the end of the hill.
h=20 m
ΔKE=-ΔPE+W_NC
KE_f-KE_i=PE_i+PE_f (W_NC = 0J because static friction does not do work)
KE_f=PE_i
½*m(v)² + ½ Iω²=mgh

½*m(v)²+1/2(2/5*m*R²)(v/R)²=mgh (I assume that a zorb ball is a uniform sphere. In reality it is not)

v²+2/5*v²=gh leads to 7/5*v²=gh

v=57∗9.8∗20=11.8m/s = 26.4 mph

This speed is pretty fast and you are also rolling in the zorb ball so it seems like a great time!