•      Friction is a force that slows down moving things.

•      Friction is what happens when any two things rub against each other.

•      These can be solid things, like our two hands rubbing together, or our skis rubbing on the snow, or a hammer hitting a nail, or they can be gases, like friction with the air slowing down our car, or liquids, like friction with the water slowing down a boat.

•      Friction happens when the rough edges of one object snag on the rough edges of another object, and some of the object’s energy has to be used to break off those rough edges so the objects can keep moving.

•      And when we rub two soft things together, like our hands, sometimes they squish into each other and get in each other's way.

•      But even completely smooth, hard things have some friction. This friction is the result of the molecules in both objects being attracted to each other.

•      There's more friction when the two objects are pushed together harder.

•      If we push our hands together, it's harder to rub them up and down.

•      If we pull the brake lever harder, our bike will stop faster.

•      Because gravity pulls harder on things with more mass, things with more mass have more friction and are harder to move.

•      A cube of iron will be harder to move than a cube of wood.

•      Two solid things usually have more friction than two liquid things, or one liquid thing and a solid.

•      That's why we slip on a wet surface more than a dry one.


For example:


Figure 23: A lady gracefully rolling her skates

•      A girl is rolling her skates, but it stops after moving a certain distance.

•      When a baseball player runs on a base, the ground pushes against the player to slow him or her down.

Figure 24: A baseball player running on a base

•      The grip on shoes is designed to create friction and thereby stop us from skidding around.

Figure 25: The grip on shoes worn by a man and a girl is designed to create friction

•      Heat is produced due to friction by rubbing our hands together.

Figure 26: A man is rubbing his hands together and they get warm due to friction produced by rubbing the hands

Figure 27: Car and bicycle create less friction on smooth road while it creates more friction on rough road

•      Generally, movement across smooth surfaces produce less friction and movement across rough surfaces produce more friction.

Figure 28: Glass and ice have smooth surfaces while a carpet has rough surface