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: B. • Sliding– solidsslide 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. 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.