Center of mass
While watching the Winter Olympics or other winter sporting events you might have wondered why skis are so long. The skis used by professionals are of top-notch quality. Researchers tinker with every detail, from the materials they are made of to the shape into which they are cut. The equipment is tested carefully because even the smallest detail 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!
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 in midair. But gravity even pulls us down when we are standing on the 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 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 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 bodies’ mass is pushed into a single point, and gravity pulls on that point, which 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.
- Baking pan or other container with sides at least one inch high
- Action figure or doll that can stand upright, preferably a heavier one (and permission to temporarily glue skis on its feet)
- School glue
- 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 the figure.
- Pour about a three-quarter-inch-deep layer of flour onto the plate. Carefully sweep the top layer with the edge of a piece of cardboard to create a flat surface.
- Imagine 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?
- Place your action figure on the flour. Pick it back up and observe. Was your prediction correct? Do you see footprints in the flour?
- Do you think it would be easy to make your action figure sink into the flour? Why?
- Place your figure back on the flour—but this time, press it down. Was your prediction correct?
- Pick up your action figure and let it stand on the flour without pushing it in. Try to knock it over to the side, then the front and the back. Is it easy to make your action figure fall?
- Dust off the feet of your action figure, then glue the skis onto its feet about halfway along the length of the skis. Let the glue dry.
- What do you think will be different if we use the action figure with skis for the tests we did before?
- Smooth the flour again with the edge of a piece of cardboard.
- What type of footprints does the action figure with skis create? Are they deeper or shallower compared with the figure without skis?
- 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?
- 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? If so, why do you think that is?
- 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 bottom of 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 the 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, whereas 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 with when the action figure wasn’t wearing the skis? This is expected.
Skis create a large contact area between the skier and the snow. Therefore, the weight of the skier—or how much gravity 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 they barely extend your feet to the side, skiers can still easily fall sideways.
Athletes competing on ice, however, use skates because they want their weight concentrated on a small area.
Remove the skis from your action figure’s feet, and peel off any remaining glue. Dispose of the flour in the compost or trash.
More to explore
Fight Slippage with Friction, from Scientific American
Slippery Science: Explore Friction by Launching Stuff, from Scientific American
Balancing Challenges, from Scientific American
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies