The Motions of Physics
The motion of physics for snowboarding can be broken down into one dimension kinematics and two dimension kinematics. One dimension kinematics cover things like inertia and friction. Two dimension kinematics covers things related to torque.
Inertia and Friction in Snowboarding
Inertia is the idea that a body in motion will stay in motion, and a body at rest will stay at rest. A snowboarder that is in motion will continue to stay in motion until his momentum is stopped by either friction or by his stopping power. However a snowboarder will stay at rest (when perpendicular to the hill) due to friction between the snow and snowboard. When traveling down a mountain, the snowboarder is almost always encountering friction. In order to lessen the effect of friction between the snow and the board, snowboarders will wax their boards, making rides much smoother as there is less friction between snow and wax. Air resistance is another factor that will slightly interfere with the speed of the rider, but there's no solution for that.
Torque and Snowboarding
Torque, which works off of momentum, is the basis for snowboarding tricks of all types. Between rail tricks and big air tricks, torque is the most important factor in executing these drills. If there isn't enough momentum in the snowboarders body before taking off a jump, he may not spin far enough fast enough to land the trick, which can often result in injury. Torque is a sort of rotational inertia. It's the basis behind every trick, even just as much as "ollieing". The other most important factor is balance. Losing your balance in mid-air is almost nearly impossible to recover from once lost in mid air. You can easily lose your balance if there isn't enough torque momentum built up before leaving the ground.
A snowboarder who doesn't have the exact amount of torque necessary can end up like this, catching the edge of his board on the landing, throwing off his balance.