If you have ever walked on slick ice, you might know how easy it is to slip and fall. Ice is very slippery because it has little friction. The surfaces of things have little bumps and scrapes that can be so small you can’t see them. These bumps and scrapes are called irregularities. Friction is caused by these irregularities getting caught on each other as two surfaces rub together. Some things like glass and ice don’t have many irregularities to get caught on, so there is little friction. Without the friction, you slip. On concrete, there are many things for your shoes to get caught on so you don’t slip.

Friction is a force that always acts in the opposite direction of the object’s motion. For instance, if you were sledding down a hill friction would be opposing the sled.

The force of friction wouldn’t actually pull the sled up the hill. It would only bring it to a stop. Once the sled stopped there would be no movement between the irregularities, so there would be no friction.

Your friend wasn’t strong enough to move the truck, so he helped you get to the gas station. Now both of you are back on the road, driving through some snow. As you go farther south, the snow turns into freezing rain, and your tires start to slip. Your friend, who you let drive, panics and jams the gas. You start moving forward a little faster, but without the friction it wasn’t much. You slowly move far enough to get back on dry pavement, where your wheels stop slipping because there is friction.

Sometimes friction would stop the sled much faster. This would happen if the force of the friction would be stronger. Two things can cause friction to change its force. One is the type of surface. If instead of the sled having a hard, smooth, plastic bottom it had something like carpet for a bottom, there would be much more friction, so the sled would come to a stop faster. The other thing is how hard the surfaces are pressed together. If a 200 pound man was riding the sled there would be much more friction than if a 50 pound child was riding it. Sometimes in the winter we put snow tires on our cars. This is because the surfaces on snow tires cause more friction than regular tires do.

Friction is sometimes very useful, but sometimes it isn’t. Without friction it would be very hard to move around. Walking would be kind of like walking on ice, but on ice there is at least some friction. Without friction you wouldn’t be able to move. Sometimes friction is not useful. In a car engine, there are many moving parts. These parts rub together, and they produce friction. The friction produced when surfaces rub together is called sliding friction. The force needed to overcome sliding friction is more powerful than the force needed to overcome fluid friction. This is why we put oil in our engines. With the oil between the moving parts, the sliding friction becomes fluid friction. With the parts oiled, it is easier to overcome the friction. Without friction, we wouldn’t have to worry about putting oil in our engines.

Rolling friction is another type of friction. We use rolling friction to reduce the force needed to overcome sliding friction. If you had two boards with marbles between them it should be pretty easy to move the top board. Sometimes in machines there are ball bearings between moving parts to reduce friction. Ball bearings are like marbles made of metal.

Remember when you were pushing the car. When it was rolling down the road, it was an example of rolling friction. If the car was in park, and you tried to push it there would be sliding friction. Now, instead of the wheels rolling on the road, they slide across it. This makes it much harder to push.

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All pictures courtesy of NASA unless otherwise noted