Image courtesy of gazetteoutthere.blogspot.com
Newton’s Law of Action-Reaction          Frictional force= ?mg(cos ?)
When one object acts exerts a force on a second object, the second object exerts an equal force in the opposite direction.  The skier’s force against the snow has an equal force of equal magnitude but in the opposite direction of the snow pushing on the skier. 

Another force that affects skiing is gravity (g = 9.81 m/s2 ).  Gravitational force =mg(sin ?).  Gravity is determined by the skier’s mass (m), and propels the skier downhill.  When moving uphill, gravity acts against the skier.  Air resistance is also dependent on skier’s shape and size and the velocity of air passing the skier.  The velocity (v=change in velocity divided by the change in time) of the air passing can be propulsive or resistive in the direction of the skier.  Adding all the external forces gives the total force that determines a skier’s motion.