INERTIA
Newton’s first law about inertia tells us that a body in motion will continue in motion unless acted upon by an external force. A snowboard without a snowboarder (or a rider doing nothing) will go down the hill in the fall line. Tat’s because the fall line is the path of least resistance or external force. An external force is needed to alter what the snowboard is doing, either turning or slowing it. Te snowboard will continue moving until the external force slows it down. Te most obvious external forces are the board- to-snow friction and wind resistance.
Te greatest external force the rider can provide is maneuvering the snowboard so it interacts with the snow. Tis can be either to slow down or turn. When a snowboard skids sideways, it increases friction and slows. If the snowboard is tilted more, it ends up interacting with the snow to slow down, and if it has a forward motion (velocity), it will make some type of turn.
From the rider’s frame of reference, this feels like being pulled out of the turn. Tis is caused by inertia as the body’s path is constantly redirected through the turn. Remember the body wants to stay in motion – that means a parallel line to the arc of the turn (angular momentum). In other words, just turning creates inertia. Riders use snowboarding skills in an attempt to manage this apparent outward pull. Tis is the centripetal force, or the force pushing back on the snowboard. It can originate from an edged snowboard in the snow, or the banked side of a mogul, or using a terrain feature such as a banked turn. Tis also explains why riding on ice is so difficult. It takes superb edging and pressure skills on the snowboard to create the necessary groove in the ice to balance out the great external forces of the turn.
MOMENTUM
When objects move, they have momentum. Momentum is calculated by multiplying the mass of an object by its velocity. Te faster a given object moves, or the greater its mass, the greater its momentum. Momentum is the essence of Newton’s laws: it takes an outside force to change a body’s momentum. We see it often on the beginner hill when we judge the space needed for our students to perform a straight run. All things being equal, a rider with less mass requires less space for a runout than a rider with more mass.
TRAJECTORY A trajectory is the path followed by a moving object. Bodies moving through the air, such as a rider off a jump, tend to follow curved trajectories as the downward force of gravity gradually overcomes the forward momentum of the object, and as wind resistance (drag) acts to slow it down. With snowboard jumps, a few other factors are able to impact the trajectory of a rider going off a jump. Broadly speaking, the rider’s momentum and angle of takeoff affect the rider’s trajectory. Even more powerful still is what he or she does with the center of mass (CM) – the central balance point of a person’s body mass – at the moment of takeoff. Similar to the movements used in up unweighting and down unweighting, the rider can either lengthen or shorten the trajectory by either extending or flexing at that moment. More extension (movement that increases the angle of a joint) equals more
24 AASI SNOWBOARD TECHNICAL MANUAL
TheSnowPros.org
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