What is friction?What is friction?FrictionFriction is the force that is the force that opposes the opposes the motionmotion of an object of an objectFriction is when Friction is when two things rubtwo things rub against against each othereach otherIf you push a book across your desk, If you push a book across your desk, the book will movethe book will moveAs the book slides across the desk, it As the book slides across the desk, it slows down and stops movingslows down and stops movingThe The initial force (F)initial force (F) is what keeps the is what keeps the book movingbook movingAs the book slides across the desk, a As the book slides across the desk, a force of frictionforce of friction (f) acts in the (f) acts in the opposite directionopposite directionThe friction slows down the motion of The friction slows down the motion of the bookthe bookThe source of the friction is theThe source of the friction is the contact contact between the surface of the book and between the surface of the book and the deskthe desk

Uses of FrictionUses of FrictionOn Earth, friction is very helpfulOn Earth, friction is very helpfulWhen you ride a bicycle, the friction between When you ride a bicycle, the friction between the road and the bicycle slows the bike downthe road and the bicycle slows the bike downAir resistanceAir resistance, a type of friction, slows down , a type of friction, slows down a parachutea parachuteWithout friction, you Without friction, you would notwould not be able to be able to slow downslow down once in motion once in motionYour own two feet keep you from slipping Your own two feet keep you from slipping and falling on the ground because of frictionand falling on the ground because of frictionSometimes friction is not helpfulSometimes friction is not helpfulFor example, metals parts in a car’s engine For example, metals parts in a car’s engine are always touchingare always touchingToo much frictionToo much friction between the metal parts between the metal parts wears them outwears them out and overheats the engine and overheats the engineIn a vacuum, there is no air resistanceIn a vacuum, there is no air resistanceThat is why all objects fall at the That is why all objects fall at the same same speed in outer spacespeed in outer space

Types of Friction:Types of Friction:– SlidingSliding – – solidssolids slide over each otherslide over each other– Ex: pushing books across your deskEx: pushing books across your desk

– RollingRolling – friction between the ground – friction between the ground and and a a wheelwheel

– Ex: RollerbladingEx: Rollerblading

– FluidFluid –– friction between friction between liquids and liquids and gasesgases

– Ex: a boat pushing through waterEx: a boat pushing through water– Ex: a paper airplaneEx: a paper airplane– Air resistance is an example of fluid Air resistance is an example of fluid

frictionfriction– Air resistance Air resistance is a force that opposes is a force that opposes

the movement of an object in airthe movement of an object in air

A.

B.

C.

Ways to Reduce FrictionWays to Reduce Friction– Add wheelsAdd wheels to change sliding friction to change sliding friction

into rolling frictioninto rolling friction– Ex: wearing roller skates instead of Ex: wearing roller skates instead of

shoes, putting a desk on wheelsshoes, putting a desk on wheels– Use lubricantsUse lubricants,, which reduce the which reduce the

amount of contact between two surfacesamount of contact between two surfaces– Ex: adding Ex: adding oiloil to an engine, to an engine, waxingwaxing a a

floor, polishing a surfboardfloor, polishing a surfboard– Examples: Examples: oil, wax, soapoil, wax, soap– Olympic swimmers wear swim caps and Olympic swimmers wear swim caps and

smooth suits to reduce friction in the smooth suits to reduce friction in the waterwater

– Sports cars are Sports cars are “aerodynamically”“aerodynamically” designed to reduce friction with the car designed to reduce friction with the car and the air. and the air.