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Friction

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Kittens_SlideTreadmill_.wmv

PBS Friction Video

 

D) Friction - force between 2 surfaces that opposes motion

Depends only on:

 

 

 

 

1) texture of each surface

 

 

What do plows have on their wheels to increase friction?

 

 

 

 

 

Chains

 

2) force holding objects together

 

 

 

 

What do race cars have mounted on their rear hood to increase their friction?

 

 

 

 

 

 

 

Spoilers increase the force between tire and road

 

 

 

 

Why do some people
put barbells in their
car trunk on a snowy
day?
 

 

 

 

 

 

Weights in a car truck
increase the tires force on the road

 

 

 

 

 

Two Kinds of Friction

 

(fs) 1) Static Friction - force of friction that must be overcome to move an object at rest.

 

 


(fk) 2) Kinetic Friction - force of friction that opposes a moving object.

 

Kinetic friction is weaker than static friction

 

fk < fs

 

 

 

 

Friction Equations

 

Static friction: fs = usFn    

 

Kinetic friction: fk = ukFn

fs = force of static friction

fk = force of kinetic friction

 

Fn = normal force (supporting force)

 

us = coefficient of static friction - "the roughness number"


u
k = coefficient of kinetic friction - "the roughness number"

 

1. Relates to texture of surfaces in contact

 

2. Calculated in lab

 

ice and skate - low uk

 

 

What do skiers do to
their skis to lower the coefficient of friction between their skis and
the snow?

 

 

 

 

 

 

Skiers put wax on the bottom
of their skis to lower the coefficient of friction

 

What is the direction of the friction at P?

 

 

 

Friction Relationships

 

 

 

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