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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









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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