NEWTON’S LAWS OF MOTION Newton’s Laws of Motion describe the relationship between the forces acting on a body
and the resulting motion. Tese universal laws help explain how skis interact with the snow. ■ Law #1: An object remains at rest or continues to move in a straight line at a constant speed if there are no unbalanced forces acting on it. Tis principle is called inertia; thus, inertia is the resistance of an object to change its state of motion or rest.
■ Law #2: When the forces acting on a body are not balanced, the net force causes the body to accelerate. Tis is expressed as F = ma. Te net force is “F,” the mass of the body is “m,” and the resulting acceleration of the body is “a.” When “F” is a lateral (sideways) force, it causes a constant change of direction instead of a change of velocity (speed), resulting in motion along a circular path. Technically, circular motion is a form of acceleration.
■ Law #3: Every force has an equal and opposite reaction force. When a skier stands on the snow, he or she is pulled down by the force of gravity (i.e., by the weight of the skier), and the snow pushes up with an equal force. Forces are balanced, that is, the net force is zero. As shown in the photo at right, the lateral force of a ski pushing into the snow during a turn (A) is balanced by the snow pushing back against the ski (B).
FORCES
Te forces that are most relevant to skiing are gravity, friction, and centripetal force.
GRAVITY
On a horizontal surface, gravity pulls the skier down upon the snow, such that he or she compresses snow beneath the skis. Te gravitational force is equal to the skier’s weight. On a slope, the force of gravity (G) is divided into two components. As shown in photo 3.5, the perpendicular component (G1) compresses the snow, while the parallel component (G2) pulls the skier downhill. Forces opposing the parallel component are friction and air resistance, but when the parallel component of gravity exceeds these opposing forces, the skier accelerates down the hill.
FRICTION
Friction is a force that opposes motion, acting in the direction opposite to forward motion. Te uphill frictional force exerted by snow upon the ski (resisting the sliding) is balanced by a downhill frictional force of the ski sliding on the snow. For example, a skier standing still on flat terrain or a very gentle slope does not move because gravity and friction (the two acting forces) are in balance. A skier exiting a chairlift and accelerating down the ramp travels forward because the component of gravity pulling the skier down the ramp is stronger than the friction exerted by the snow.
TheSnowPros.org CHAPTER 3: PHYSICS OF SKIING 51
PHOTO 3.4: The force of gravity (A) is balanced by the opposing force from the snow (B).
A B
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