In the real world, however, it is fortunately as impossible to get away from friction as it is to get away from the other laws we have tried to imagine as being turned off. There is always some friction, or rubbing, whenever anything moves. A bird rubs against the air, the point of a spinning top rubs against the sidewalk on which it is spinning. Your shoes rub against the ground as you walk and so make it possible for you to push yourself forward. The drive wheels of machinery rub against the belts and pull them along. There is friction between the wheels of a car and the track they are pushing against, or the wheels would whirl around and around uselessly. [Illustration: FIG. 24. Hockey is a fast game because there is little friction between the skates and the ice.] But we can increase or decrease friction a great deal. If we make things rough, there is more friction between them than if they are smooth. If we press things tightly together, there is more friction than if they touch lightly. A nail in a loose hole comes out easily, but in a tight hole it sticks; the pressure has increased the friction. A motorman in starting a trolley car sometimes finds the track so smooth that the wheels whirl around without pushing the car forward; he pours some sand on the track to make it rougher, and the car starts. When you put on new shoes, they are so smooth on the bottom that they slip over the ground because of the lack of friction. If you scratch the soles, they are rougher and you no longer slip. If you try to pull a stake out of the ground, you have to squeeze it harder than the ground does or it will slip out of your hands instead of slipping out of the ground. When you apply a brake to an automobile, the brake must press tightly against the axle or wheel to cause enough friction to stop the automobile. There are always two results of friction: heat and wear. Sometimes these effects of friction are helpful to us, and sometimes they are quite the opposite. The heat from friction is helpful when it makes it possible for us to light a fire, but it is far from helpful when it causes a hot box because of an ungreased wheel on a train or wagon, or burns your hands when you slide down a rope. The wear from friction is helpful when it makes it possible to sandpaper a table, scour a pan, scrub a floor, or erase a pencil mark; but we don't like it when it wears out automobile tires, all the parts of machinery, and our clothes. EXPERIMENT 17. Hold a nail against a grindstone while you turn the stone. Notice both the wear and heat. Let the nail rest lightly on the stone part of the time and press hard part of the time. Which way does the nail get hotter? Which way does it wear off more quickly? Run it over a pane of glass and see if it gets as hot as it does on the grindstone; if it wears down as quickly.