External Forces

The term force describes something that causes a skier to undergo a change in speed or direction. It is simply anything that pushes or pulls on an object. This chapter discusses the interaction between the combination of skier and skis and the snow surface.

Two categories of forces are experienced when skiing:

Internal forces are generated by the skier through muscular-skeletal movements. These types of forces are covered in The Four Movements.

External forces include gravity, friction and the reaction forces from the snow. These are forces that act upon a skier. A skier must manage these forces through proactive and reactive movements to stay in balance.

Managing the Forces

A skier and the equipment they are wearing (boots, clothing, etc) is a skier’s mass. This term is defined as a measurement of the quantity of matter an object possesses. This is slightly different from the term weight, which is a measurement of gravity’s effect on the object. An object with greater mass will have a larger weight because gravity is a constant.

The skier’s centre of gravity is defined as the mean location of the weight of a body. For our purposes it is the theoretical point in the body where all mass acts as if it is concentrated. As a skier moves, this point may be inside of or outside of the body (see above image). Where this point is located determines where the skier is balanced on the skis. When we describe the location of the skier’s centre of gravity we describe its relationship to the base of support. This is defined as the portion of the ski or skis under the foot or feet. A skier standing still on flat ground is in balance when their centre of gravity is over their base of support from a lateral and fore/aft perspective.

A skier standing still on two flat skis, with their centre of gravity balanced over the middle of their base of support will be evenly balanced over two feet. The result of this is felt as pressure against the soles of both feet. Pressure is represented by the equation P=F/a where P = pressure, F = force, and A = area (units are N/m2). It is important to understand that if the contact area is reduced or the force is increased the pressure is increased. For example, when a skier lifts their right ski off the snow they reduce the surface area (a), but force (F) – the skier’s weight – remains unchanged, therefore pressure (P) has increased on the left ski.

The relationship of these two reference points will become increasingly important as the skier moves from the static example above, to being in motion (see above image).

In general, the force that puts a skier in motion is gravity. A skier in a straight run is pointed down a shallow slope and therefore begins to move. Since gravity (G) always pulls us towards the centre of the earth, we will break the effects of gravity into a few components to show how it puts the skier into motion.

G1 is the component of gravity that puts the skier into motion. G1 is parallel to the slope. G2 keeps the skier in contact with the terrain. G2 is perpendicular to the slope. As skiers straight run down a gentle slope they begin to speed up. This is an example of acceleration. Acceleration is defined as a change in velocity, or speed, in a particular direction, of an object. Acceleration happens when there is an external force applied to an object. In the case of the skier in a straight run, gravity causes the skier to accelerate. Further examples of acceleration are changing direction, speeding up as skis are pointed down a slope, and slowing down through skidding or braking.

If two skiers of different sizes are both straight running on the same gentle slope with a natural run out, the smaller skier will begin to slow down and stop in a shorter distance than the larger skier. This is because the larger skier, the one with greater mass, has more momentum. Momentum is defined as “quantity of motion”. It is the product of the body’s mass and velocity or speed. The greater momentum, the more a body will resist change in motion.

Friction

The force that resists the motion of sliding on or through a surface is defined as friction. Simply put, it is friction which slows a skier down.

A skier experiences two types of friction:

• Friction from the surface of the snow
• Friction from the air – also called wind resistance

The direction of friction is generally the opposite of the skier’s momentum. In a straight run the force of friction is parallel to the skis; in a hockey stop it is perpendicular to the skis (see diagram below). 


When skiers wax their skis they are attempting to lower the amount of friction acting upon the skis to allow them to travel faster. A ski racer wearing a speed suit is doing so to lessen the slowing effect of wind resistance. A freestyle aerialist, on the other hand, often wears looser fitting clothing in order to use wind resistance to control or alter their rotation in the air.

When skiers attempt a spin or 360 they are creating rotational momentum. This is defined as a quantity of rotation of a body. Once they begin to spin they will continue to spin until an outside force, such as friction, acts on them. Similarly, they will continue to spin in the same direction until an outside force acts on them to change direction. When performing a 360 in the air a skier gains rotational momentum through takeoff and continues to spin in the same direction until landing. While rate can be affected, the direction of rotation will remain constant. As in the example of a straight run, given everything else is equal, a larger skier will spin for longer than a smaller skier.

In a ski turn we move both linearly (momentum) and about an axis (rotational momentum). Any change in the velocity of a body in motion, speeding up, slowing down or change of direction is due to the application of an outside force. That outside force can come from friction, wind resistance or deflection from terrain or another body.