November 30, 2008
It's that time of year again for Canadians. Winter is here. It's time once again to impress your friends with your fashionable thermal underwear and wool socks. Fortunately, now that winter is here, so are all of the outdoor activities that make it worthwhile, like downhill skiing. Let's take a quick look at the physics of snow and why skis can slide on it. Keep in mind that while I'm using skis as an example, the same physics applies for anything else you dare ride to the bottom of a snow-covered hill.
Did you know? We have seasons because the earth's axis of rotation is tilted. As the earth orbits the sun, the tilt causes an uneven distribution of the sun's energy during different times of the year. During summer in Canada, the Northern Hemisphere is more directly exposed to sunlight, causing longer days and more heat absorption than in the winter.
Snow forms in the atmosphere when water vapor goes through a process called sublimation. This means that the water vapor condenses and turns directly into its solid form — ice. As tiny amounts of crystalline ice form in this way, beautiful patterns emerge, and we call these objects snowflakes. After snowflakes hit the ground, they lose their patterned shape and become much more like little grainy balls of ice that stick together. This happens because of compaction from the weight of the snow and from heat rising up from the ground.
So what's special about snow that enables us to hop on a pair of skis and slide downhill? The answer lies in the f word, friction that is. Friction is a force that resists motion. There are two different kinds of friction, static friction, and kinetic friction. When something is static, it isn't moving, so static friction is the friction between two things in contact that aren't moving relative to each other. If you put a book on a surface like a table, and start to tilt the table, the book initially won't start to slide. This is because static friction is preventing the book from moving. If you increase the angle of the table by tilting it more, all of a sudden the book will start to slide. When this happens, the force on the book from gravity that acts downward along the table has overcome the force from static friction that was preventing it from moving.
Did you know? Amazingly, what you may have heard about snowflakes is true, if you took a look at every snowflake ever made, chances are you wouldn't find two that are exactly alike.
Once the book starts to slide, kinetic friction, the friction involved with motion, becomes important. Generally speaking, the "rougher" the surface of the table, the higher its kinetic friction will be. The more friction there is, the more resistance there is to motion, and the slower the book will slide down the table. When a hill becomes covered in snow, we can ski down it because the static and kinetic frictional forces become much less than they were when the hill was covered in grass or dirt. This paints a simple picture of why we can ski on snow, but like all things in science, the closer you examine what's happening, the more interesting things become.
Did you know? The study of surfaces sliding against each other, like skis sliding on snow, is called tribology, and it is involved with everything from lipstick to bearing design.
If you take a closer look at what happens to snow as an object slides across it — you will find that it actually melts! Kinetic friction produces heat (rub your hands together and try it out). When you slide across the snow, some of it melts because of the heat produced by this frictional force. The small amount of water from the melted snow lubricates the surface of the skis in contact with the snow the same way that oil reduces wear on moving engine parts - by decreasing friction. If it's really cold (below about -25°C), this won't happen, and the snow will remain dry. Dry snow has much more friction than wet snow, so below -25°C you won't be able to go downhill as fast as you could on snow above this temperature. If it gets that cold, it's probably a good idea to stay indoors and avoid having to use your thermal underwear and wool socks anyway!
Dave Lind and Scott P. Sanders, The Physics of Skiing: Skiing at the Triple Point. AIP Press, New York, 1996.