Friction

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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

 

 

 

 

 

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

Friction on Ramp in terms of m, theta

f = U[mgcosq]

 

ice and skate - low uk

 

Friction on horizontal surface

Level Force

fs = uFn = umg

Friction on horizontal surface

Tilted Push, F, q, m

fs = uFn = ?

 

Fn = Net downward force on lawn mower

 

Fn = [mg + Fsinq]

fs = u[mg + Fsinq]

What if mower was pulled?

 

 

 

fs = u[mg - Fsinq]

 

 

 

 

Friction Relationships

 

 

 

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