What is friction? Friction is the force that opposes the motion of an object Friction is when two things rub against each other If you push a book across your desk, the book will move As the book slides across the desk, it slows down and stops moving The initial force (F) is what keeps the book moving As the book slides across the desk, a force of friction (f) acts in the opposite direction The friction slows down the motion of the book The source of the friction is the contact between the surface of the book and the desk

Uses of Friction On Earth, friction is very helpful When you ride a bicycle, the friction between the road and the bicycle slows the bike down Air resistance, a type of friction, slows down a parachute Without friction, you would not be able to slow down once in motion Your own two feet keep you from slipping and falling on the ground because of friction Sometimes friction is not helpful For example, metals parts in a car’s engine are always touching Too much friction between the metal parts wears them out and overheats the engine In a vacuum, there is no air resistance That is why all objects fall at the same speed in outer space

Types of Friction: –Sliding – solids slide over each other –Ex: pushing books across your desk –Rolling – friction between the ground and a wheel –Ex: Rollerblading –Fluid – friction between liquids and gases –Ex: a boat pushing through water –Ex: a paper airplane –Air resistance is an example of fluid friction –Air resistance is a force that opposes the movement of an object in air A. B. C.

Ways to Reduce Friction – Add wheels to change sliding friction into rolling friction – Ex: wearing roller skates instead of shoes, putting a desk on wheels – Use lubricants, which reduce the amount of contact between two surfaces – Ex: adding oil to an engine, waxing a floor, polishing a surfboard – Examples: oil, wax, soap – Olympic swimmers wear swim caps and smooth suits to reduce friction in the water – Sports cars are “aerodynamically” designed to reduce friction with the car and the air.