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)

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

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

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

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

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.