# Why Are Skis So Long?

## Summary

Key Concepts
Weight, pressure, force, balance, center of mass
Credits
Sabine De Brabandere, PhD, Science Buddies

## Introduction

While watching the Olympic Winter Games, you might have wondered why skis, which are featured in several events, are so long. The skis used at the Olympics are of top-notch quality. Researchers tinker with every detail, from the materials they are made of to the shape they are cut in. The equipment is tested carefully, as details like these can give an athlete the leading edge on the slope. But why would you put planks under your shoes in the first place? Do this activity to find out!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

## Background

Earth’s gravity pulls us all the time; it makes us return to the ground when we jump, and it makes objects fall when we release them. But gravity even pulls us down when we are standing on a hard floor! You see this when you sit on a mattress: your weight, or how much Earth pulls on you, pushes the mattress down. As a result, the mattress squeezes together where you sit. If you stand on the mattress, you should notice that it compresses more than if you lie down or sit on it. When you are standing, all your weight pushes the small area where your feet touch the mattress. If you sit or lie down, your weight is distributed over a much larger area, so for each square inch that your body is touching the mattress, there is a much smaller force, and the mattress does not become as compressed.

Snowshoes use this concept to make you sink less into fresh, fluffy snow. Snowshoes create a larger contact area with the snow, so your weight pushes less on each square inch of it. The strength of each push per square inch of contact area is what scientists call pressure, and snowshoes reduce that pressure.

A larger contact area with the snow or floor has more advantages. Gravity acts on us as if all our body's mass is pushed into one point, and gravity pulls on that point. This point is called the center of mass. If this point is located above the contact area we make with the floor, we will not fall over. The larger the contact area, the easier it is to keep our center of mass above it, and the more stable we will feel.

## Materials

• Deep plate. A glass baking tray or baking pan work well, too.
• Flour
• Action figure or doll that can stand upright—preferably a heavier one—and permission to temporarily glue skis on its feet.
• Cardboard
• Scissors
• School glue

## Preparation

1. To make skis for your action figure, cut two identical rectangles out of cardboard. The length should be a little shorter than the height of your action figure. The width should be about double the width of the feet of your action figure.
2. Pour a layer of flour about ¾ inch deep onto the plate. Sweep the top layer with the edge of a piece of cardboard to create a flat surface.

## Instructions

1. Imagine that the flour is fluffy snow. What would happen if you stepped onto this fluffy snow? Will the same happen when you place your action figure there?
2. Place your action figure on the flour. Pick it back up and observe. Was your prediction correct? Do you see footprints in the flour?
3. Do you think it would be easy to make your action figure sink into the flour? Why?
4. Place your figure back on the flour, but this time, press it down. Was your prediction correct?
5. Pick up your action figure and let it stand on the flour without pushing it in. Try to knock it over to the side, the front, and the back. Is it easy to make your action figure fall?
6. Glue the skis onto the feet of your action figure. The feet should be about halfway along the length of the skis. Let the glue dry.
7. What do you think will be different if we use the action figure with skis for the tests we did before?
8. Smooth the flour again with the edge of a piece of cardboard.
9. What type of footprints does the action figure with skis create? Are they deeper or shallower compared to the action figure without skis?
10. Is it easier or harder to make this action figure sink into the flour? Why would that be beneficial when you want to slide over the snow?
11. Can you knock over the action figure with skis as easily? Is knocking it so it falls to the front or the back as easy as making it fall to the side? Why would this be so?

EXTRA: Freeze a plastic tray of water to make a frozen pond and see what happens if your action figure stands on ice. Does it leave footprints on the ice? Do people sink into ice like they sink into the snow? Could you still benefit from wearing skis when walking on ice?

EXTRA: Look up the characteristics of arctic animals. Do they have bigger paws than similar animals in warmer climates so they do not sink as much into the snow?

EXTRA: Give your action figure ice skates. To make one ice skate out of cardboard, you will need two small rectangles a little longer and wider than your action figure’s foot. One rectangle represents the shoe and will be glued to the foot of your action figure. The other rectangle represents the blade. Attach it vertically to the shoe (the first rectangle). This makes one skate. Repeat for the second skate. What impressions do skates make on the flour and on ice? Can you easily knock over an action figure balancing on skates? Why do you think this is the case? Why would athletes competing on snow wear skis, while athletes competing on ice would wear skates?

## Observations and Results

Were the action figure’s footprints shallower when it was wearing skis? Was it harder to push your action figure with skis into the flour, and was it harder to knock it over to the front and the back compared to the action figure without the skis on? This is expected.

Skis create a large contact area between the skier and the snow. Therefore, the weight of the skier—or how much the earth pulls on him—is spread out over a larger area. The skier presses less on each square inch of snow, and instead of sinking into it, he or she can glide over it. You could also feel this when you pushed your action figure into the flour. Without the skis, it was easy. With the skis, your force was distributed over a large area. It was more difficult to make the action figure sink into the flour.

Skis also help with stability. It is easier to keep your balance when you have a bigger contact area. Think about how you tend to fall forward or backward when you stand on your toes; standing on flat feet is easier. Similarly, skis extend the feet, making it hard to fall forward or backward, but as skis barely extend your feet to the side, skiers can still easily fall to either side.

Athletes competing on ice use skates because they want their weight concentrated on a small area. This gives them a better grip on the ice, making it easier to push off and jump or gain forward speed. Skates are also far shorter, which allows for tight turns and gracious moves that would never be possible with skis.