External Forces, CoG & BoS
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.
Browse below to see some of the terms and concepts that are good to understand, develop and enjoy when skiing and playing with the forces.
CoG
CoG is the Centre of Gravity.
The skier’s centre of gravity is for our purposes the theoretical point in the body where all mass acts as if it is concentrated.
View of CoG when viewing side on
BoS
BoS is the Base of Support.
The base of support is/are the skis on the snow
Snow Pushes a Skier Round a Turn
Sir Isaac Newton identified a set of laws that involve forces and acceleration, momentum and inertia.
First Law
an object in a state of motion tends to stay in that state of motion unless an external force acts upon it
So a skier will not change direction unless a force acts upon them. This means that the skis need to interact with the snow so that the snow can push the skier around the turn.
Second Law
states that the direction of the force is the same as the direction of the acceleration
So if the direction of the external force (force of the snow pushing back) is to the right, then the skier will turn to the right.
Third Law
every action has an equal and opposite reaction and all forces involve two objects and the force acts with equal strengths but in opposite direction on both.
In a ski turn the two objects are the skier (and skis) and the snow. The force from the snow pushes on the ski causing the skier to change direction.
CoG v BoS
The faster the skier travels through a turn, the greater the centripetal force and (apparent) centrifugal force will act upon the skier.
The more force acting on a skier during a turn, the more the skier will need to move their centre of gravity (CoG) further inside the turn relative to their base of support (BoS).
Moving the CoG inside the turn will allow the skier to create a round turn at higher speeds.
Friction
A skier experiences two types of friction:
- Friction from the surface of the snow
- Friction from the air – also called wind resistance
CoG In A Turn
With speed and grip, two factors that are required to make a turn, the result is that centripetal force is present.
The centripetal force acting towards the centre of the turn means skiers must move their centre of gravity inside the path of their base of support to stay in balance.
Lateral movement of the centre of gravity towards the centre of the turn is required to balance with the forces.
