I will have to apologize in advance, but the snow outside has me really excited, so the next few blog posts may very well be about winter physics.  One thing that I am very excited to do now that it is officially winter, is sledding/tubing.  As I am not a very good skier or snow boarder, I opt for the easier version; sitting and allowing gravity to do the rest.  All of these downhill, winter activities, however, are possible thanks to conservation of energy and friction.  Conservation of energy is necessary in order to attain speed as one goes down the hill.  To start, when one is at the top of the hill, all energy is in the form of potential energy (U=mgh).  However, as one goes down the hill, the height from the bottom decreases, so the potential energy decreases.  As a result, the kinetic energy (K=.5mv^2) increases in order to compensate for the decrease in potential energy (conservation fo energy).  Another factor that seems important in these activities is friction.  The coefficient of friction for ice and snow is lower than that of say grass, and it makes sense.  You can't go skiing or sledding down the side of a grassy mountain because the force of the friction up the hill caused by the grass is greater than the force of the skier or sledder (mgsintheta).  So, it is necessary to have a material (ice or snow) that has a lower coefficient of friction in order to get going down the hill.  I can't wait to get out on the slopes and let friction and energy do their jobs!