1. What is Friction?/Lesson 6
A force that opposes motion between two surfaces that are in contact.
Causes a moving object, such as a ball, to slow down and stop.
Occurs because the surface of any object is rough (microscopic hills and valleys)

2. Source of Friction
When two surfaces are in contact, the hills and valleys of one surface stick to the hills and valleys of the other surface.
The amount of friction between two surfaces depends on:
the force pushing the surfaces together
Increase in force = increase in friction
Heavier weight = increase in friction
the roughness of the surfaces.
Rougher surface=more hills and valleys=increase in friction

3. Types of Friction Kinetic “moving” Static “not moving”
Friction between moving objects
Sliding
Applying the brakes of a bicycle
Rolling
Anything that has wheels
Static “not moving”
When a force is applied to an object but does not cause it to move
Overcome by applying a large amount of force

4. Friction: Harmful or Helpful
Car tires to ground =
moving car
Car brakes to wheels = stopped car
Pencil to paper = leaves a mark (your handwriting)
Keeps you from slipping and falling when walking
Increased temperature in car parts can wear the parts down
Holes in your socks
Wind and water eroding topsoil that nourishes plants

5. Reducing Friction Lubricants applied to surfaces to reduce friction
Examples: motor oil, wax, and grease.
Changing the type of friction
Sliding kinetic friction to rolling kinetic friction
Example: ball bearings in in-line skates and bicycles
Make surfaces that rub against each other smoother.
Rubbing the surfaces with sandpaper

6. Increasing Friction Make surfaces rougher
Examples: sand scattered on icy roads, baseball players wearing textured batting gloves
Increase the force pushing the surfaces together
Example: pressing sandpaper harder when sanding a piece of wood.