slip out from under him and he falls on his face. He lies in the same
spot no matter how he wriggles and squirms. If he tries to push with
his hands, they slip over the rough ground more easily than they now
slip through air. He cannot push sideways enough even to turn over. If
there happens to be a rope within reach and one end is tied to a tree,
he might try to take hold of the rope to pull himself along. But no
matter how tightly he squeezes, the rope slips right through his hands
when he starts to pull. If, however, there is a loop in the rope, he
can slip his hand through the loop and try to pull. But the knots with
which the rope is tied immediately come untied and he is as helpless
as ever.

Even if he takes hold of a board fence he is no more successful.
The nails in the board slip out of their holes and he is left with a
perfectly slippery and useless board on the ground beside him for a
companion. As it grows cold toward evening he may take some matches
out of his pocket and try to start a fire. Aside from the difficulty
of his being unable to hold them except by the most careful balancing
or by shutting them up within his slippery hands, he is entirely
incapable of lighting them; they slip over the cement beneath him or
over the sole of his shoe without the least rubbing.

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 trol