The Gravity Dance – Physics 101 For Ski Bums

Skiing is all about manipulating forces...

Gravity, Momentum & The Ground Reaction Force:

Gravity is the universal force that pulls two objects together. Here on this planet, it's what makes us feel like we're stuck to the Earth's surface.

Obviously gravity is super important to skiing. When we stand on a slope covered in snow gravity is trying really hard to pull us towards the center of the earth. 

Fortunately the slope pushes back on us, and thanks to the slippery sticks on our feet, friction (another force that helps us stick) is reduced and we slide down the hill.

Now we've got some momentum!

Earth’s gravity pulls on your mass towards its’ centre

The slope pushes back so gravity pulls you down the hill

Depending on how we manipulate our skis and align our body segments whille sliding, the snow pushes back on us to either slow our mass down, change it's direction, launch it into the air... or do a variety of other strange and wonderful things.

C.O.M. vs B.O.S.

Another important concept to understand in skiing is the relationship between the Centre of Mass and the Base of Support.

The Centre of Mass is the 3 dimensional balance point of an object. In a skier it's located somewhere just above the pelvis.

To make things more complicated, due to all our movable body parts, our Center of Mass actually moves around depending on what position we're in.

Move arm to one side... center of mass moves too!

Flex only at the ankles... COM moves forward

Bend only at the hips... center of mass moves forward

Flex both knees... COM moves back

The Base of Support refers to the area beneath an object or person that includes every point of contact that the object or person makes with the supporting surface.

In a skier this is the base of your skis and the area bewteen where they contact the snow. When you plant your pole this expands your base of support.

When we're just standing around without skis on, our base of support is the bottom of our feet.

Turning Forces:

Okay so we've created a steering angle by twisting our skis aganist our momentum... and we've tipped our skis on edge to get a little grip. Suddenly we start changing direction.

Centripetal Force helps us turn by pushing an object to towards the center of an arc.

Centrifugal Force (an apparently ficticious force) is caused by a constant change in momentum. When centripetal force pushes your skis towards the centre of the arc, your centre of mass wants to continue along it’s path (inertia). It gives you that sensation of being pulled to the outside of an arc.

To balance while traveling on an arc, we need to find the happy place between gravity and centrifugal force.

This lean into the turn is called 'inclination.' The turning forces vary with speed and turn shape. As a result, the amount of inclination required also varies with speed and turn shape.

Centripetal force makes us travel in an arc

Inertia - Are the oranges moving… or the car?

The equal & opposite reaction to centripetal force

Inclination to balance against a combination of forces

Inclination - merry go round

So why the physics lesson?

Well, if I were to sum up ski technique in one sentence, I would say we need to coordinate movements with all our different body parts so that our base of support directs our center of mass.

There you go... the secret to skiing!

Newton’s Laws of Motion

  • Law 1 - A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.
  • Law 2 - When a body is acted upon by a force, the time rate of change of its momentum equals the force.
  • Law 3 - If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.

Some Handy Physics Definitions

  • Force – A push or pull on an object.
  • Torque - A twisting force. The rotational equivalent of linear force.
  • Pressure – A measurement of force per unit area.
  • Weight – The force with which a body is attracted towards earth.
  • Net force – The sum of all forces that act upon an object.
  • Momentum – The quantity of motion of a moving body. Expressed mathematically as a product of mass × velocity.
  • Inertia – A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.
  • Acceleration – The rate of change of velocity with respect to time.
  • Speed  – The rate at which an object is moving
  • Velocity – The rate and direction an object is moving
  • Impulse – A sudden force acting on a body producing a finite change of momentum.

Outsmart your ski instructor and troubleshoot your own ski technique

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nancygreene
nancygreene
2 years ago

I live with two Physics teachers so this is what our dinner conversations sound like. Very interesting!

Lucas Kott
Lucas Kott
7 months ago

educational & straight to the point …great demos!

Alan Robb
Alan Robb
5 months ago

To start with gravity is straight down. When you start sliding down a hill it can be spit into two components, one parallel to the slope and one perpendicular. Your weight you feel on your feet is the component perpendicular to the slope. The force pushing you down the hill is the component parallel to the slope. As the slope gets steeper you become lighter and the force pushing you down the hill greater. What happens at the limit of the slope?

T brown
T brown
4 months ago
Reply to  Alan Robb

Do you feel weight? Or do you feel pressure?

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