3.  This section introduces and describes friction.  You will explore the types of friction and study examples of each.

4.  Friction is a force that opposes motion between 2 surfaces that are touching.  Friction occurs because the surface of any object is rough.  When 2 surfaces are in contact, the hills and valleys of 1 surface stick to the hills and valleys of the other surface.

5.  These surfaces have more microscopic hills and valleys, therefore the rougher the surface, the greater the friction.  Increased friction is more effective at stopping things.  If the surfaces are smooth, there is less friction.

6.  The amount of friction also depends on the force pushing the surfaces together.  If force is increased, the hills and valleys of the surfaces can come into closer contact.  This causes friction between the surfaces to increase.  Less massive objects exert less force on surfaces than more massive objects do.

7.  Sliding Friction is very effective at opposing the movement of objects and is the force that causes objects to stop moving.  Examples: moving a dresser, sledding, brakes on a bicycle or car, writing with a piece of chalk.

8.  If the same heavy dresser were on wheels, you would have an easier time moving it, this is an example of rolling friction.  The force of rolling friction is usually less than the force of sliding friction.  Therefore, it is generally easier to move objects on wheels than it is to slide them along the floor.

9.  Walking on a wet floor is harder than walking on a dry floor, why?  On a wet floor the sliding friction between your feet an the floor is replaced by fluid friction between your feet and the water.  Fluid friction is less than sliding friction, so the floor is slippery.  Fluid friction opposes the motion of objects travelling through a fluid.

10.  When a force is applied to an object but does not move, static friction occurs.  The object does not move because the force of static friction balances the force applied.  Static friction disappears as soon as an object starts moving, and then another type of friction immediately occurs.

11.  Without friction, tires on cars could not push against the ground to move the car and brakes could not stop the car. Without friction a car is useless.  Friction between moving engine parts increases their temperature and causes them to wear down.  Friction causes the holes in your socks, erosion. However without friction you would slip and fall when you walk.

12.  One way to reduce friction is to use lubricants.  Lubricants are substances that are applied to surfaces to reduce the friction between them.  Friction can also be reduced by switching from sliding friction to rolling friction.  Another way to reduce friction is to make surfaces that rub together smoother.

13.  One way to increase friction is to make surfaces rougher.  Examples: sand on icy roads, batting gloves  Another way is to increase the force pushing the surfaces together.