Friction IT’S CALLED “DRIFTING”. Questions to ponder What is friction? What factors affect friction? What don’t? How are WE affected by friction?

FrictionIT’S CALLED “DRIFTING”

Questions to ponder

What is friction?

What factors affect friction? What don’t?

How are WE affected by friction?

Does there always need to be movement for friction to occur?

What would happen if there was no friction?

What is friction?

GENERAL DESCRIPTION:Friction is a force that RESISTS the

relative motion between two SURFACES that are in contact.

It opposes motion.

What is friction?

More than meets eye…Friction is a VERY COMPLEX phenomenon. Every simple statement made about friction can be

COUNTERED with specific examples that show otherwise.

NO universal agreement as to what causes it. We have a few useful MODELS that work (have PREDICTIVE POWER) up to a point.

1. Coulomb Model of Friction Developed by Charles Augustin De-

Coulomb (named then unit for charge after him!)

A surface, when zoomed in, has many bumps: PEAKS and VALLEYS.

These peaks INTER-LOCK and cause resistance to movement. Two hair brushes pushed past

each other. Your knuckles dragging against

each other.

1. Coulomb Model of friction

Toilet Paper and Rubber as seen through a scanning electron microscope.

RUBBERTOILET PAPER

2. Adhesion Theory of Friction Tiny bumps are found in all material that

come into contact    MICROWELDS are formed

Small attractive forces exist that cause “BONDING” between materials.

Generally considered a better model, but the math is very complicated and it fails to make predictions at a certain point.

What factors affect friction? Types of surfaces involved

Roughness – Generally, the rougher the two surfaces, the HIGHER the frictional force.

Smoothness – generally, the smoother two objects become, the LOWER the frictional force

Stickiness – glue, tape, or any sort of adhesive tends to INCREASE the intermolecular forces between two objects, which increase friction.

Whether the surface is wet/icy – Water and ice fill in the irregularities in a surface making it SMOOTHER.

What factors affect friction? The force pushing the surfaces together.

F1 Racers have a spoiler/air-foil on the back to create a DOWNWARD FORCE.

This increases the CONTACT force between the tires and the road and the friction as well.

Temperature As temperature increases, the force

of friction DECREASES. Hot brakes = bad !

Why are race car tires so much wider than our regular car tires?

http://science360.gov/obj/video/7fdabae3-a9f2-4ed0-8059-16da6ad2ea72/science-speed-friction-heat

Wide vs. thin tires Surface area DOES NOT DIRECTLY affect friction. Wide tires are for COOLING

Remember! Temperature DECREASES friction! If you want to cool down your food, you spread it across your

plate increasing the SURFACE AREA. The traction increase is due to a number of factors:

Increase the likelihood that the tire is in CONTACT with the ground.

Tires are made of a sticky rubber that increases its GRIP. Design of the race-car’s body allows for a larger downward force.

Car Tires and Tread

Tire tread refers to the GROOVES in a car tire.

Prevent HYDROPLANING Water that would usually create a smooth

surface is PUSHED OUT and BACK.

Types of Friction

Does it take more force to get something moving than it does to keep it moving?

To keep an object moving at a constant velocity over a rough surface, you need to exert a constant force that is the SAME MAGNITUDE as the frictional force.

Types of Friction An object at REST always needs MORE FORCE

to get it moving. We call this STATIC FRICTION .

It will resist ANY force until a point, Ff

max ,after which it will start moving.

Once moving, KINETIC FRICTION comes into play. It is the friction experienced by an object IN MOTION.