“Autumn...the year's last, loveliest smile." - William Cullen Bryant -

FRICTION IS A FORCE THAT IS CREATED WHENEVER TWO SURFACES MOVE OR TRY TO MOVE ACROSS EACH OTHER. FRICTION ALWAYS OPPOSES THE MOTION OR ATTEMPTED MOTION OF ONE SURFACE ACROSS ANOTHER SURFACE. FRICTION IS DEPENDANT ON THE TEXTURE OF BOTH SURFACES. FRICTION IS ALSO DEPENDANT ON THE AMOUNT OF CONTACT FORCE PUSHING THE TWO SURFACES TOGETHER.

PHYSICISTS USUALLY TALK ABOUT 4 KINDS OF FRICTION. THE FIRST THREE TYPES INVOLVE CONTACT WITH SOLID SURFACES: 1)STATIC FRICTION 2)SLIDING FRICTION 3)ROLLING FRICTION

THINK ABOUT STATIC FRICTION AS THE AMOUNT OF FORCE THAT HAS TO BE APPLIED TO OVERCOME THE RESISTANCE TO MOTION OF TWO OBJECTS IN CONTACT.

THE AMOUNT OF STATIC FRICTION IS DETERMNED BY TWO FACTORS: 1)HOW ROUGH THE CONTACT SURFACES ARE. 2)THE AMOUNT OF NORMAL FORCE PUSHING THE TWO SURFACES TOGETHER. IN THE FIRST CASE, THERE IS NO SUCH THING AS A COMPLETELY SMOOTH SURFACE, ALTHOUGH ICE COMES PRETTY CLOSE.

IN TRUE ROLLING MOTION, YOU WOULD HAVE SOME FRICTION AT FIRST. THIS WOULD KEEP THE WHEEL FROM SLIPPING. ONCE ROLLING MOTION IS ACHIEVED, YOU WOULD HAVE LITTLE FRICTION DUE TO THE ONE TO ONE CONTACT OF THE WHEEL SURFACE AND THE CONTACT SURFACE. IN ACTUAL CASE, YOU HAVE SOME DEFORMATION OF BOTH SURFACES, SO YOU HAVE ADDITIONAL FRICTION.

IN SURFACE TO SURFACE CONTACT, WE USE LUBRICANTS TO REDUCE THE FORCE NEEDED TO OVERCOME FRICTION – EITHER STATIC OR SLIDING FRICTION. THERE IS NO SUCH THING AS A COMPLETELY SMOOTH SURFACE.

THE FOURTH TYPE OF FRICTION IS FLUID FRICTION. ANYTIME YOU ARE MOVING AN OBJECT THROUGH A FLUID, YOU HAVE TO EXPEND ENERGY (FORCE) TO PUSH ASIDE THE FLUID MOLECULES. YOU ENCOUNTER THIS WHEN YOU MOVE OBJECTS IN A FLUID OR PUSH A FLUID THROUGH A PIPE.

THIS RESISTANCE TO FLOW OR MOVEMENT IN A FLUID AN BE THOUGH OF AS VISCOSITY (LIQUID) OR AIR RESISTANCE (GAS). IN BOTH CASES, IT IS A FORCE TO BE OVERCOME.