What is friction?

  It is hard to push a piece of sandpaper over a rough piece of wood. There is, we say, a great deal of friction between the sandpaper and the wood. It is hard to pull a big box loaded with coal along a concrete sidewalk. There is a great deal of friction between the box and the sidewalk. Whenever two surfaces rub together there is some friction. But the fric­tion is greater if the surfaces are rough than if they are smooth.
  Friction produces heat. An eraser used to rub out a pencil mark gets warm. Some­times there is a "hot-box" in the wheel of a train because there has been too much friction between the wheel and the axle. Scratching a match on sandpaper makes the match so hot that it catches on fire.
  Putting oil or grease between the two surfaces that are rubbing together is one way of making friction less. Using rollers or wheels or ball bearings is another.
  Friction makes our coats wear out at the elbows. It makes us have to buy new automobile tires and new shoes. It costs us a great deal for oil and grease and wheels and ball bearings.
  But it is a good thing that there is some friction. Without it no knot would stay tied. We could not fasten anything together with nails, because the nails would not hold. We could not go anywhere in a train or an automobile. The wheels would spin round and round in the same place. We could not even walk about. Floors and sidewalks would be far slicker than ice.