What is the max video fps Nikon D5100?
I think you're out of luck. 
Official specs say 30fps max, and I can't find any documentation of a possible hack. 
http://www.nikonusa.com/en/Nikon-Products/Product/Digital-SLR-Cameras/25478/D5100.html
The most widely distributed slow-mo camera is probably the iPhone 5s-- I bet you could borrow one to play around with.