Ice is weird. Seriously. It’s one of very, very few solids in the universe that floats on top of its liquid form instead of sinking. Seriously. Toss a rock into lava, and it will (eventually) sink. Steel bar into molten steel? Sink. Salt crystal into molten salt? Well, you get the idea.
One of the other weird things about ice is how things are able to slide across it so easily. Try sliding a hockey puck across dirt or grass. Or ice skating on concrete or wood. It’s just not really gonna work right. You need ice for easy sliding and skating; if you want to slide or skate on another solid, you’re gonna need to modify your tools. This is because different surfaces exhibit different levels of friction, which is to say they resist motion across them more or less. Today’s experiment will let you see how friction differs between ice and other materials.
- Something smooth and flat (like a hockey puck)
- (If you have a hockey puck, that would be best)
- (You know what, just go buy a hockey puck)
- A cookie sheet
- A floor
- Fill the cookie sheet with water and then put it in the freezer until the water has completely turned to ice.
- While your water is freezing, test your puck (or whatever) out on your floor. Hardwood, carpet, a cement basement; whatever. The more, the merrier! Give it a good push. How does it move? Does it slide easily? Does it run out of energy and slow down quickly? Does it tumble? Test it out a few times, so you get a feel for how it moves over a normal surface.
- Take the cookie sheet out of the freezer and spread a small amount of warm water on top of the ice. Make sure it’s just enough to melt the very top layer of ice and leave it nice and slippery.
- Put your puck on one end of the cookie sheet and give it a good push just like you did before on the floor. What happens to it as it tries to move across the slippery surface? How does it move? What do the wheels do? How is it different from what you observed before? Try it a few times to gather lots of observations.
Rub your hands together really fast. What happens? Do you feel them getting warmer? That’s friction in action.
Friction is a force that resists things moving against each other. This means that when two things, like a puck and ice (or floor, or turf), are touching and at least one of them is moving, friction wants them to stop moving. Often times friction generates heat, like when you rubbed your hands together.
Carpet, concrete, wood, linoleum, etc. all exert more friction on objects sliding over them than ice, as you likely saw in your experiment. A hockey puck is smooth to minimize the friction it exerts on the surface, and ice is smooth as well (at least right after the Zamboni), but that’s only part of the story here. As long as the temperature is above -4° F (-20° C), there is a thin layer of liquid water on top of the ice. See? Weird! This thin layer of liquid is what really lowers the friction between the ice and the puck, allowing the puck to slide on over. And it’s why we made sure we put a tiny bit of warm water on your cookie sheet of ice: just in case your freezer was set for below -4° F.
This interaction among the puck, the ice, and that tiny water layer, are why a hockey puck can’t be used for street hockey (and why ice skates only work on ice; they glide on that water layer, too. And actually create their own under the blades. But that’s a different story entirely). Instead, other solutions had to be invented. Some street hockey pucks have build-in rollers. Sometimes street hockey just uses a ball. And, of course, the blades on ice skates were replaced with the wheels on inline skates so they can move easily over non-ice surfaces.
Want more Saturday Science? See all of our at-home activities on the blog or on Pinterest. Also, be sure to put the science of sports to the test at the Riley Children's Health Sports Legends Experience. You can rule the rink at the Hockey Experience!